bluuebunny/crossref_metadata_2025 huggingface dataset:
Renewal of deep water in the Red Sea during 1982–1987
Authors: Woelk Sofie , Quadfasel Detlef
Published: February 2004

Between October 1982 and May 1983 the deep water in the northern Red Sea was ventilated through slope convection. The deep water became cooler, fresher, and more oxygenated. Dense bottom water formed in the Gulf of Suez during winter cooling sank down the continental slope, thereby entraining near‐surface and intermediate depth waters. From the application of a one‐dimensional plume model, a vertical transport of 0.58 × 106 m3 s−1 over a period of 7 months was estimated. Long‐term time series of atmospheric heat fluxes show that such a major convection event can occur every 4–7 years. The corresponding renewal time of the Red Sea deep water is thus of the order of 40–90 years, in agreement with previous estimates.
Decadal‐scale trends in mechanisms controlling meridional sea surface temperature gradients in the tropical Atlantic
Authors: Wagner Richard G.
Published: February 2004

The meridional gradient of sea surface temperature (SST) in the tropical Atlantic hydrostatically controls the atmospheric pressure gradient, thereby steering the latitude position of the Intertropical Convergence Zone (ITCZ), which affects rainfall in adjacent land areas. This study investigates the mechanisms controlling long‐term changes in meridional SST gradients, using surface and subsurface ship observations from 1951–1990. SST forcing from surface heat fluxes and entrainment are scaled by monthly climatic‐mean mixed‐layer depth. Observed SST tendency and contributions from various forcing mechanisms are computed for the 1951–1990 mean and trend. A substantial warming trend in the surface waters of the tropical South Atlantic is concentrated in the austral summer months of February–March, while SST in the tropical North Atlantic increased in boreal summer (August–September) and decreased in winter. As a consequence of the strengthened interhemispheric SST gradient in February–March, the position of the ITCZ was displaced southward, and rainfall in northeast Brazil increased over 1951–1990. A southward displacement of the ITCZ during boreal summer, accompanying warming between the equator and the ITCZ contributed to the decrease of rainfall over the west African Sahel. Strongest warming trends in the respective summer hemisphere imply an amplification of both seasonal cooling during winter and warming during summer in each hemisphere. During September–February a trend toward intensified northeast trade winds results in increased seasonal cooling due to latent heat transfer, whereas weakened southeast trades reduce latent heat loss, intensifying seasonal warming. For March–August a dominant mechanism forcing enhanced seasonal cooling in the South Atlantic is associated with entrainment as mixed‐layer depth increases during the transition to winter; this process acts to dilute warm anomalies developed during summer.
Observation of autumn freeze‐up in the Beaufort and Chukchi Seas using the ERS 1 synthetic aperture radar
Authors: Winebrenner D. P. , Holt B. , Nelson E. D.
Published: February 2004

We present observations of the transition in sea ice backscattering signatures from late summer into early winter in the Beaufort and Chukchi Seas during 1991 and 1992, using data from the ERS 1 synthetic aperture radar (SAR). We employ both analyzed surface temperature fields and direct observations of near‐surface temperature from drifting buoys. Consistent with previous surface‐based observations, backscattering from sea ice surviving at the end of summer increases strongly and rapidly when temperatures fall below freezing for the final time in the autumn, apparently because of increased volume scattering from bubbles in the upper part of the ice. Areally averaged backscattering sometimes overshoots typical winter multiyear ice values by 1–2 dB, before settling back over a period of approximately a week. The backscattering‐temperature link forms a sufficient basis for an algorithm to estimate retrospectively, from time series of SAR images, the date of freeze‐up (defined here to be the date on which all liquid water in the bubbly upper layer of surviving sea ice freezes and remains frozen for the duration of the ensuing autumn and winter). We present a prototype algorithm and use it to estimate freeze‐up dates in nine Lagrangian cells in the Beaufort Sea during the autumn of 1992. We observe a 12‐day spread in freeze‐up dates between latitudes of approximately 73°N and 82°N, with dates in the northernmost cells of August 29–30 and those in the southernmost cells of September 7. Two cells at latitudes of 75°N–77°N appear to freeze‐up earlier than cells further north, but present uncertainties limit the significance of this observation.
Cosmonaut polynya in the Southern Ocean: Structure and variability
Authors: Comiso Josefino C. , Gordon Arnold L.
Published: February 2004

Along the far eastern margin of the Weddell Gyre is a persistent feature in the middle of the ice pack which we previously reported and called the Cosmonaut polynya. A study of polynya occurrences from 1973 to 1993 reveals that since 1986 the polynya has become more active with an average size of about 7.2 × 104 km2 and an average location at 52°E and 65°S. Satellite observations indicate that the polynya has recurred several times during winter in recent years with intervals ranging from a few days to a few weeks. The centroid of the polynya varies only slightly with each formation during the year and from one year to another, suggesting a controlling influence of the ocean and bottom topography that may be initially induced by wind. The daily time series indicates two primary modes of formation: one that is initiated in the early winter during a storm at a site usually preceded by an embayment of the ice edge and another that occurs during midwinter often preceded by a coastal polynya event adjacent to Cape Ann. The Cosmonaut polynya region is characterized in this study by compression of the westward flowing coastal current and the eastward flowing southern edge of the Antarctic Circumpolar Current. Following the principle of conservation of potential vorticity, vertical stretching of the water column would ensue, enhancing upwelling. Such a process accelerates the injection of relatively warm salty deep water into the surface layer, inhibiting sea ice growth and causing the polynya formation. This theory appears to explain the general behavior of the polynya in terms of frequency, duration, size, and location.
Sea level from temperature profiles in the tropical Pacific Ocean, 1975–1982
Authors: Reverdin Gilles , Kaplan Alexey , Cane Mark A.
Published: February 2004

We investigate how well the 1975–1992 sea level interannual variability in the tropical Pacific is captured by dynamic height from temperature profiles. For each temperature profile, a surface dynamic height relative to 300 m is estimated, assuming a constant temperature‐salinity relationship. After multiplication by a latitudinally varying factor and the removal of a seasonal cycle, the dynamic height deviations fit the tide gauge sea level variability to within the sampling errors, except at a few sites near the equator west of the date line, where surface salinity variability is large. The dynamic height data are assimilated into a wind‐forced linear numerical model of the sea level in the tropical Pacific, applying a Kalman filter in a space of reduced dimension. A limited number of empirical orthogonal functions of the unfiltered run (1975–1992) define the reduced space, into which the Kalman Filter covariance evolution calculation is done [Cane et al., 1996]. Experiments indicate that results are better with 32 functions than with a smaller number but are not improved by retaining more functions. The resulting analyzed fields of sea level are compared to withheld dynamic height estimates from moorings, sea level data from tide gauges, and sea level analyses made with the same Kalman filter formalism applied to tide gauge measurements. The comparisons to observations suggest that the temperature profiles were usually sufficient to constrain the monthly analyzed fields to be close to the observed sea level with errors typically less than 3 cm near the equator. The comparison to tide gauge sea level reveals that this analysis is often more accurate than the analysis of tide gauge sea level data with which it shares many characteristics. Near the equator west of the date line, salinity variations are large and their neglect in estimating dynamic height has a negative impact on the analysis. The analyzed signal is underestimated in the southwest Pacific and at more than 20° off the equator. The reanalysis of the temperature data done with a primitive equation model at the National Meteorological Center (NMC) [Ji et al., 1995; Enfield and Harris, 1995] does not share this problem. On the other hand, NMC reanalysis (RA4) departs more from the observations elsewhere, although more data were included than in our analysis.
Sea ice and oxygen isotopes in Foxe Basin, Hudson Bay, and Hudson Strait, Canada
Authors: Tan Francis C. , Strain Peter M.
Published: February 2004

The summer distribution of sea ice meltwater and river runoff and the surface circulation in Foxe Basin, Hudson Bay, and Hudson Strait have been studied by oxygen isotope techniques. On the basis of oxygen isotope and salinity relationships, the reduction of surface salinity in summer in Foxe Basin is predominantly due to sea ice meltwater. River runoff and sea ice meltwater contribute equally to the summer surface layer in northern Hudson Bay. The isotopic data clearly show the flow of Foxe Basin water through Roes Welcome Sound into northwestern Hudson Bay, but no evidence for this outflow is found in the passages connecting Hudson Bay and Hudson Strait. Oxygen isotopes indicate that recent river runoff is concentrated in a narrow band along the south side of Hudson Strait. The wide variety of relationships between oxygen isotope ratios and salinity is explained by the known ice dynamics and the seasonal variability predicted from ice melting and freezing models.
Tidal‐induced large‐scale regular bed form patterns in a three‐dimensional shallow water model
Authors: Hulscher Suzanne J. M. H.
Published: February 2004

The three‐dimensional model presented in this paper is used to study how tidal currents form wave‐like bottom patterns. Inclusion of vertical flow structure turns out to be necessary to describe the formation, or absence, of all known large‐scale regular bottom features. The tide and topography are treated as a coupled system, and the potential free instabilities in this system are investigated. A linear stability analysis is used to study the dynamics of bed forms. This yields information on the role of physical mechanisms; such information cannot be obtained by direct numerical nonlinear simulation. The basic state represents a horizontal elliptical flow, veering in vertical direction, over a flat bottom. It is shown that in such a system, modes can grow which have characteristics of both tidal sand banks and sand waves. The model predictions are compared with observations and show good agreement.
On the mesoscale interaction of lead ice and floes
Authors: Hopkins Mark A.
Published: February 2004

The plasticity of the Arctic ice pack depends on its granular nature, in particular on the size and distribution of areas of thin ice and open water surrounding multiyear ice floes. The paper begins with construction of a mesoscale (10–100 km) granular model of the central Arctic ice pack. The mesoscale model is based on a dynamic particle simulation in which individual multiyear ice floes and surrounding parcels of first‐year ice are explicitly modeled as discrete, convex polygons in a two‐dimensional domain. Deformation of the domain produces areas of localized failure and areas of open water. The areas of localized failure are modeled as pressure ridging events using the results of numerical experiments performed with a computer simulation of the ridging process. The paper focuses on the results of numerical experiments performed with the mesoscale model. In the experiments the model ice pack is biaxially deformed at constant strain rates. The principal strain rates are varied to create deformation states ranging from pure shear to uniform compression. The results define the shape and magnitude of the plastic yield surface, the strain rate vectors associated with points on the yield surface, the partition of energy dissipation between ridging and in‐plane sliding, and the changes in the ice thickness distribution associated with various deformation states.
Lateral transport of settling particles in the Ross Sea and implications for the fate of biogenic material
Authors: Jaeger John M. , Nittrouer Charles A. , DeMaster David J. , Kelchner Charlotte , Dunbar Robert B.
Published: February 2004

The Ross Sea, Antarctica, with its high rates of primary productivity and biogenic accumulation, provides an important location to test the validity of a one‐dimensional particle‐settling model. As part of an interdisciplinary field project performed from 1990 to 1992 to examine cycling and accumulation of biogenic matter in the Ross Sea, water‐column particulate and current data were collected at three sites. At each of the sites, a current meter and sediment trap were placed 240 m below the water surface, and a similar set of instruments was located 40 m above the seabed. The moorings were deployed for 1‐ to 2‐years duration. The current‐meter records showed that the speed of flow in the southwestern Ross Sea is relatively slow (< 20 cm s−1) and current direction is highly variable. Currents in the south‐central Ross Sea have moderate speeds (<40 cm s−1) and less directional variability. The northwestern Ross Sea has the strongest flows (>50 cm s−1) and least variability in direction. To examine the validity of a one‐dimensional approximation for fluxes of biogenic material, two models were developed to determine the net displacement of particles settling through the water column. Current‐meter data and particle‐settling characteristics were incorporated in both models. One model produced a time‐varying, linearly interpolated current field between the moorings in which particle advection was evaluated. The second model used time‐averaged progressive‐vector plots to estimate lateral particle advection. Results show that particles are displaced the least at the southwestern site (<20 km), moderate distances at the south‐central site (<50 km), and the greatest at the northwestern site (>50 km). The pattern in displacement trends correlates well with observed sediment types and accumulation rates at each site. A one‐dimensional model for the settling of biogenic material is most applicable at the southwestern site and least applicable at the northwestern site.
Seasonal variations of the Antarctic coastal ocean in the vicinity of Lützow‐Holm Bay
Authors: Ohshima Kay I. , Takizawa Takatoshi , Ushio Shuki , Kawamura Toshiyuki
Published: February 2004

During the period of 1990–1992, year‐round oceanographic observations were conducted in the vicinity of Lützow‐Holm Bay, East Antarctica. It was found that the thickness of the Winter Water (WW) layer, characterized by a cold fresh oxygen‐rich water, exhibits its maximum in the austral fall (typically 500 m) and its minimum in the austral summer (typically 350–400 m). The associated density variation of the water column explains only about one third of the seasonal variations in sea level at the coast, which suggests a large seasonal variation in barotropic coastal flow. Prominent freshening occurs in the WW layer during fall. This appears to be caused by the accumulation of WW, whose upper portion is freshened in the preceding summer. These seasonal variations appear to occur every year. We propose that the seasonal variation in the WW layer is mainly caused by the seasonal variation in the wind over the coastal ocean. In fall the prevailing easterly wind intensifies, which increases the Ekman convergence of WW in the coastal ocean, while in summer the opposite occurs.
Oscillating turbulent flow over very rough surfaces
Authors: Giménez‐Curto Luis A. , Lera Miguel A. Corniero
Published: February 2004

The forces on the seabed in shallow water under waves influence near‐shore transport processes. However, the actual nature of these forces is not yet fully understood. Sleath [1987] simultaneously measured horizontal shear force per unit area and Reynolds stress in oscillating turbulent flow over granular beds with the striking result that maximum Reynolds stress was significantly less than total shear force per unit area of bed. Trying to explain these measurements, we use a formulation which considers two kinds of flow perturbations, namely turbulent fluctuations and some disturbances due to boundary irregularities. The resulting spatially averaged Reynolds equations contain in particular two terms which do not appear in the smooth bed case: the force due to the mean momentum flux for boundary disturbances, here called “form‐induced stress,” which owes its existence to the voracity of the disturbed motion, and the force exerted by the roughness elements on the fluid. The “jet regime” as introduced by Giménez‐Curto and Corniero [1993] for steady flow is extended to oscillatory flow. In this regime, pressure drag on roughness elements is the fundamental force acting on the boundary, and form‐induced stress due to vorticity generated by flow separation from bed irregularities becomes the leading stress, thus providing an explanation for Sleath's measurements by means of a physical mechanism which was already envisaged by Longuet‐Higgins [1981] for two‐dimensional rippled beds. A simple expression is derived for the friction coefficient which is subsequently compared with extensive series of measurements in the laboratory for granular beds as well as for rippled surfaces, showing an excellent agreement.
A note on spatial modes in longshore current shear instabilities
Authors: Dodd N. , Falqués A.
Published: February 2004

Spatially growing disturbances in an alongshore current are considered. In many other types of shear flows these spatially growing modes describe the initial development of instabilities more satisfactorily than do the more commonly considered temporally growing modes. Here the spatial stability theory is applied to these wave‐driven, semi‐infinite types of flows for the first time. Both model [Thornton and Guza, 1986] and measured [Reniers et al., 1994] longshore current and beach profiles are analyzed, using both the temporal and spatial theories. Results of the two theories are compared using the approximate relations of Gaster [1962]. For the Thornton and Guza profile, predictions of the spatial theory and those from the transformed temporal theory are seen to be extremely close. For the profile of Reniers et al., which exhibited destabilization and spatially growing instabilities, the two sets of predictions are again very close. This implies that the simpler temporal stability theory, which has been used exclusively in previous studies, may be applied even when the spatial theory would clearly be more appropriate (such as in the experimental study examined here) by transforming the temporal predictions via Gaster's relations. Moreover, it appears that the predicted wavelength and period of the fastest growing mode (FGM) of untransformed temporal theory will be fairly good indicators of the same quantities for spatial modes and perhaps may be used as such when estimates of the group velocity from the temporal theory cannot be made with much certainty, such as when points are very sparse. It is shown that the basic structure of the fastest growing disturbance or mode (FGM) is similar for both theories for the Reniers et al. profile, although it would be a mistake to rely on the details of the linear eigenfunctions given by temporal theory.
Spatial‐scale analysis of hydrographic data over the Texas‐Louisiana continental shelf
Authors: Li Yongxiang , Nowlin Worth D. , Reid Robert O.
Published: February 2004

On the basis of hydrographic data collected by the Texas‐Louisiana Shelf Circulation and Transport Processes Study (LATEX A) and on earlier cruises, we examined the energetic scales of spatial variability over the Texas‐Louisiana continental shelf. Shelf‐scale spatial reference fields were sought to represent the general distributions of circulation and water properties over the shelf at the time of the observations. Various methods were explored for determining such reference fields of potential temperature, salinity, and geopotential anomaly at the sea surface relative to 70 dB. Spatial reference fields obtained from mean May fields and from polynomials fitted to individual May cruise data were compared. On the basis of those comparisons, quadratics were selected to fit property distributions from individual cruises and so to yield reference fields. Smaller‐scale anomaly fields were obtained by removing the reference fields from the observed distributions. Calculation of correlation versus separation distance based on these anomaly fields then allowed estimation of spatial scales of anomaly fields for cross‐shelf and along‐shelf transects. The zero‐crossing scale and the Gaussian decay scale are shown to be essentially the same, and the zero‐crossing scale is used. The principal results for the anomaly scales are (1) cross‐shelf scales over the western shelf are shorter (order 15 km) than those in the eastern and central regions (order 20 km), (2) along‐shelf spatial scales are of the order of 35 km, (3) there is no significant difference in cross‐shelf scales at the surface, middepth, and bottom, and (4) along‐shelf scales are essentially the same over the western and eastern regions of the shelf, over the midshelf (50‐m isobath) and along the shelf break (200‐m isobath), and at different depths along the 200‐m isobath. The same spatial scales are found when using data with spatial resolution of 1–10 km cross shelf and 10–20 along shelf to obtain the anomaly fields, so the data resolution used is adequate to represent the scales. The variances of the observed (shelf‐wide) salinity, temperature, and geopotential anomaly are greater cross shelf than along shelf. The variance of the cross‐shelf anomaly fields is around 10% of the shelf‐wide fields; that of the along‐shelf anomaly fields is about 35% of that in the shelf‐wide fields. The analysis of scales when grouped by season did not show persuasive evidence of seasonal variation.
Measurements of in situ 14C concentrations in Greenland Ice Sheet Project 2 ice covering a 17‐kyr time span: Implications to ice flow dynamics
Authors: Lal D. , Jull A. J. T. , Burr G. S. , Donahue D. J.
Published: February 2004

We present results of measurements of 14C in CO and CO2 phases for selected Greenland Ice Sheet Project 2 firn and ice samples up to a depth of 1838 m (16.6 kyr B.P.). The results show that 14C in ice consists of both in situ‐produced cosmogenic 14C, which can be either in the form of CO or CO2, and trapped atmospheric 14CO2. In agreement with our previous work on studies of 14C concentrations in Antarctic firn and ice samples and some GISP2 samples, we found that the cosmogenic in situ‐produced 14C (from spallation of oxygen nuclei) is greater than the trapped atmospheric 14C component. Also, we can show the total 14C amounts in ice are consistent with an efficient retention of the in situ variety of 14C in older ice where we expect lower average temperatures at the time of formation. The firn samples appear to have lost much of the 14C, probably during storage. The ice accumulation rates are estimated after due corrections for the CO2 content of trapped air and “secondary” 14C production in ice during storage at altitude. These values are in good accord with the recent model estimates of Cutler et al. [1995], except for two samples, at depths 1518.5 m (9740 years B.P.) and 1698.5 m (12,360 years B.P.). In the first case, the in situ 14C concentration is significantly higher than that predicted by either of the models, by a factor of about 2. In the second case, the estimate of Cutler et al. [1995] is a ∼ 50% lower accumulation rate, but the 14C result is consistent with adjacent samples. A possible reason for the first discrepancy may be the decreased atmospheric pressure at that time at the site, as suggested by D. Raynaud et al. (unpublished manuscript, 1995).
Thin ice thickness from satellite thermal imagery
Authors: Yu Y. , Rothrock D. A.
Published: February 2004

Sea ice thickness is an important parameter in the surface energy exchange and mass balance of the polar oceans. Remote sensing offers the opportunity to monitor thin ice routinely. We combine measurements of surface temperature from an advanced very high resolution radiometer (AVHRR) with a thermodynamic ice growth model to estimate thin ice and its thickness distribution during the cold season. In a Beaufort Sea area of perennial ice we find that thin ice, up to a meter thick, had a concentration of 18% in December 1990 and 3% in April 1991; thin ice concentrations near the ice edge in the Greenland Sea range from 20% to 50%. These AVHRR‐derived estimates generally compare well with thickness observations from upward looking sonar. Given the uncertainties in all the model inputs estimated from AVHRR, such as radiative and turbulent heat flux components and surface albedo, we find that ice thickness is estimated with an uncertainty of roughly 50%. The uncertainty in the cumulative thickness distribution is about 3% for ice less than 20 cm thick and about 9% for ice around 1 m thick; these magnitudes are significantly smaller than the variations in thin ice observed by satellite in different months and regions. We conclude that AVHRR is capable of resolving the regional and seasonal variations in thin ice to a level of accuracy useful both for establishing a climatology and for testing ice models.
Localized convection in rotating stratified fluid
Authors: Whitehead J. A. , Marshall J. , Hufford G. E.
Published: February 2004

We study the convective overturning of a rotating stratified fluid in the laboratory. Convection is induced from the surface of a salt‐stratified fluid by the introduction of salty fluid over a circular area. The external parameters are buoyancy forcing of strength, B0, applied over a circular area of radius Rs, the rotation rate as measured by f, ambient stratification N, and the depth H. The experiments are motivated by physical scaling arguments which attempt to predict the length and velocity scales of the convective chimney as it adjusts under gravity and rotation and breaks up through baroclinic instability. The scales of interest include the number, size, and typical speeds of the fragments of the broken chimney, the final depth of penetration of the convective mixed layer, and the total volume of convectively produced water. These scales are tested against the laboratory experiments and found to be appropriate. In this idealized problem we have found the depth of penetration depends only on the size and strength of the forcing and the ambient stratification encountered by the convection event; it does not depend explicitly on rotation. The implications of the work to deep water formation in the Labrador Sea and elsewhere are discussed. Finally, the study has relevance to the role and representation of baroclinic eddies in large‐scale circulation of the ocean.
Estimating the full‐scale flexural and compressive strength of first‐year sea ice
Authors: Kovacs Austin
Published: February 2004

Sea ice salinity, density, and temperature data were used to develop new methods for determining the bulk brine volume and porosity of sea ice floes. Methods for estimating full‐thickness ice sheet strength, based on large‐scale field tests, are presented. The relationships among bulk sea ice properties, strain rate, and strength are illustrated. A new constitutive equation was developed for predicting the full‐thickness horizontal compressive strength σc of first‐year sea ice as a function of the applied strain rate and bulk porosity in the form
where parameters B2, n, and m are about 2.7×103, 3, and −1, respectively, and
and ϕB are the ice strain rate and ice floe bulk porosity of sea ice, respectively. An example of the first‐year sea ice indentation force against a 90‐m‐wide structure is given. Estimating sea ice strength based on remote ice conductivity measurements is also discussed conceptually.
Assimilation studies of open‐ocean flows: 2. Error measures with strongly nonlinear dynamics
Authors: Gunson James Reginald , Malanotte‐Rizzoli Paola
Published: February 2004

In a companion paper by Gunson and Malanotte‐Rizzoli [this issue], the problem of estimating boundary and initial conditions for a regional open‐ocean model from sparse data is addressed using the adjoint method. Here the estimation of error covariances for the estimated boundary and initial conditions and interior fields, in the presence of strongly nonlinear dynamics, is investigated. The evaluation of the full error covariance matrix for the estimated control variables from the inverse Hessian matrix is presented along with its dependence upon the degree of nonlinearity in the dynamics. Major new results here obtained are the availability of off‐diagonal covariances, the successful calculation of error covariances for all boundary and initial conditions, and the estimation of errors for interior fields. The role of the Hessian matrix is assessed in gauging the sensitivity of the estimated boundary and initial conditions to the data.
Ocean‐atmosphere exchange of methyl chloride: Results from NW Atlantic and Pacific Ocean studies
Authors: Moore Robert M. , Groszko Wayne , Niven Stewart J.
Published: February 2004

Measurements of methyl chloride concentrations and saturations in the NW Atlantic and Pacific Ocean from 40°N to 40°S are presented. Labrador Sea waters, analyzed in July, were at or below saturation, while warmer waters south of the Gulf Stream and all Pacific samples were consistently supersaturated. We find no correlation between methyl chloride concentrations and chlorophylla, but much of the variation in methyl chloride partial pressure appears to be accounted for by variations in sea surface temperature. We estimate ocean‐atmosphere fluxes of methyl chloride, using exchange velocities for spot wind speeds and climatological wind speeds. These indicate that none of the waters we sampled support the magnitude of fluxes which have hitherto been considered to be emitted by the ocean. A simple extrapolation yields a global efflux of 0.8 to 1.1 × 1010mol yr−1(0.4 to 0.6 Tg yr−1) supplied by warm waters and a high‐latitude influx of 0.2 to 0.5 × 1010mol yr−1(0.1 to 0.3 Tg yr−1). The net global ocean source to the atmosphere is 0.3 to 0.9 × 1010mol yr−1(0.2–0.4 Tg yr−1), which accounts for 5 to 12% of the total required to balance loss by reaction with hydroxyl radicals. Unless its atmospheric reactivity has been overestimated, we conclude that other sources of methyl chloride make major contributions to its atmospheric budget.
Winter snow cover on sea ice in the Weddell Sea
Authors: Massom Robert A. , Drinkwater Mark R. , Haas Christian
Published: February 2004

Measurements of snow thickness, temperature, salinity, density, and stratigraphy acquired during the 1992 Winter Weddell Gyre Study are presented. Results indicate that the winter snow cover on sea ice in the Weddell Sea is extremely variable. Extreme fluctuations in Antarctic synoptic conditions (air temperature, precipitation, humidity, and wind speed) occur during the austral winter. They result in unique modifications and additions to the snow layer during the aging process and act to stabilize an otherwise easily wind‐redistributed shallow snow cover and develop well‐packed drift features. The latter occur even over relatively undeformed areas of sea ice and have a significant localized effect on the snow thickness distribution. Significant variability in snow grain size (mean 2.73±3.12 mm) and density (0.32±0.09 g cm−3) is observed as a result of cyclical switches between high‐ and low‐temperature gradient metamorphism. Multiple icy layers indicate multiple thaw‐freeze events. One such event occurred during a 3‐day station, during which the air temperature rose by 22°C in 12 hours (to approximately 0°C). This paper also examines mechanisms for flooding of the snow‐ice interface, including snow loading. Even where the latter is not a factor, the layer of snow immediately above the snow‐ice interface is commonly damp and saline (>10‰). Limitations in the data set are discussed, and comparisons are drawn with other experiments.
A numerical simulation of flow at Fieberling Guyot
Authors: Beckmann A. , Haidvogel D. B.
Published: February 2004

A numerical sigma‐coordinate ocean circulation model is used to investigate tidally forced flow near Fieberling Guyot. The aim is to reproduce the observed currents and to identify the dominant physical mechanisms that lead to the complex three‐dimensional flow fields at this tall and steep seamount in the northeast Pacific. Our very high resolution simulation (with 500 m horizontal and less than 20 m vertical grid spacing in the vicinity of the seamount summit) was performed with the newest version of the terrain‐following sigma‐coordinate primitive equation model (SPEM). As in previous more idealized studies, the seamount was placed in the center of a uniformly rotating periodic channel and forced by diurnal barotropic tides. The exponential initial stratification, the tidal forcing amplitude, and its orientation were chosen based on observations from the Fieberling area. Three major characteristics of the flow observed at Fieberling Guyot are reproduced both qualitatively and quantitatively: diurnal currents reach about 20 cm s−1, a twentyfold amplification relative to the tidal flow away from the seamount; the time‐mean anticyclonic flow speeds are close to the observed 10 cm s−1 maximum azimuthal velocity; and the spatial structure of this vortex shows a maximum at about 6 km radius between 400 and 600 m depth, clearly lifted off the bottom. The time‐mean flows are found to be maintained by diurnal waves of a mixed type: the net motion shows characteristics of both bottom‐intensified seamount trapped waves and vertically propagating vortex trapped waves. While the former are mainly responsible for setting up a time‐mean anticyclonic flow along the upper flanks at the bottom, the latter are needed to redistribute the momentum and time‐mean density, thereby reproducing the observed structure with a flow maximum off the bottom. An analysis of the energy, momentum, and density fluxes shows that rectification depends critically on the eddy fluxes that balance the time‐mean downwelling over the seamount summit and upper flanks. The results of sensitivity and parameter studies are utilized to further interpret the role of individual physical mechanisms and the time‐mean dynamical balances.
The flow field of the subtropical gyre of the South Indian Ocean
Authors: Stramma L. , Lutjeharms J. R. E.
Published: February 2004

The mean state of the transport field of the subtropical gyre of the South Indian Ocean has been derived for the upper 1000 m from selected historical hydrographic data. The subtropical gyre in the southwestern Indian Ocean is stronger than the flow in the other two oceans of the southern hemisphere. Most of the water in the South Indian gyre recirculates in the western and central parts of the basin. In the upper 1000 m the eastward transport of the South Indian Ocean Current starts with 60 Sv in the region southeast of South Africa. Between the longitudes of 40° and 50°E about 20 Sv of the 60 Sv recirculates in a southwest Indian subgyre. Another major diversion northward occurs between 60° and 70°E. At 90°E the remaining 20 Sv of the eastward flow splits up, 10 Sv going north to join the westward flow and only 10 Sv continuing in a northeastward direction to move northward near Australia. Near Australia, there is indication of the poleward flowing Leeuwin Current with a transport of 5 Sv. In the central tropical Indian Ocean between 10°S and 20°S, about 15 Sv flows to the west. The western boundary current of this subtropical gyre consists of the Agulhas Current along the east coast of southern Africa. Its mean flow is composed of 25 Sv from east of Madagascar and 35 Sv from recirculation in the southwest Indian subgyre south of Madagascar, with only 5 Sv being contributed from the Mozambique Channel. A net southward transport of 10 Sv results for the upper 1000 m of the South Indian Ocean. In contrast to the triangular shape of the subtropical gyre in the South Atlantic, probably caused by the cross‐equatorial flow into the North Atlantic, the area influenced by the subtropical gyre in the South Indian Ocean is more rectangular.
Sea surface temperature variability in Panamá and Galápagos: Extreme temperatures causing coral bleaching
Authors: Podestá Guillermo P. , Glynn Peter W.
Published: February 2004

We examined associations between warm sea surface temperature (SST) anomalies and coral bleaching in the Galápagos Islands and the Gulf of Panamá, in the tropical eastern Pacific Ocean. Interannual SST variability is dominated by the El Niño‐Southern Oscillation phenomenon at Galápagos, whereas only strong events have an SST signature in Panamá. We explored various SST‐related metrics potentially associated with bleaching occurrence: maximum absolute SST, SST anomaly, and the combined effect of intensity and duration of both SST anomalies (described via a “degree days” index) and high SST events. In Galápagos, three Niño years (1983, 1987, and 1992) coincided with bleaching. These were the top three years in both maximum annual SSTs and degree days values. In Panamá, bleaching in 1983 coincided with high maximum SSTs and high degree days. In contrast, no bleaching was detected in 1972 despite high values of both quantities. We found all temperature‐related metrics to be highly correlated, and it was impossible to isolate their effects.
Energetics of borelike internal waves
Authors: Henyey Frank S. , Hoering Antje
Published: February 2004

The net integrated energy flux into a train of internal waves is evaluated in a two‐layer model. The nonzero value for this integral results from the difference in the stratification between the initial and final state, similar to the energy supply to a surface bore. We apply this expression to waves measured by Wesson and Gregg [1988] in the Strait of Gibraltar and to waves measured by Farmer and Smith [1980] in Knight Inlet. We find the energy supply to be important to the energetics, but the data do not allow a definitive test of the conjecture that the primary energy balance is between this supply and dissipation. We contrast our conjecture to the solitary‐wave considerations of Bogucki and Garrett [1993].
Long‐period ocean heat storage rates and basin‐scale heat fluxes from TOPEX
Authors: Chambers D. P. , Tapley B. D. , Stewart R. H.
Published: February 2004

Over 3 years of TOPEX altimeter data have been used to estimate annual heat storage and long‐period heat storage rates in 1° grids over the global oceans (65°S to 65°N). The mean annual heat storage rates computed from the TOPEX data agree with those computed from monthly‐mean temperatures to within 30 W m2. The accuracy of long‐term heat storage rates is estimated to be less than 10 W m2 over mid and low latitudes. Regional heat storage rates are larger than this error in many regions, most notably in the tropical Pacific and Indian Oceans and in the northern Atlantic Ocean. The heat storage changes associated with the 1991–1993 El Niño event are evident and agree with values computed from in situ measurements collected by the Tropical Ocean‐Global Atmosphere program Tropical Atmosphere Ocean array moorings. The heat storage rates inferred from TOPEX data are also shown to qualitatively agree with sea surface temperature rates computed over the same time period. Finally, heat storage rates are integrated over ocean gyres to estimate atmosphere‐ocean heat fluxes. Results suggest that from 1993 to 1995 the North Atlantic and the southern hemisphere gained heat from the atmosphere at a rate of between 1.5 and 2 W m2, while the North Pacific either lost heat or maintained zero net flux. However, the estimated error on this measurement is of the order of 1.5 W m2 indicating the true flux could be from 0 to 3 W m2. Both the TOPEX and temperature measurements suggest that the North Atlantic gained more heat per unit area than the North Pacific between 1992 and 1995.
Evaluation of forest canopy models for estimating isoprene emissions
Authors: Lamb Brian , Pierce Thomas , Baldocchi Dennis , Allwine Eugene , Dilts Steve , Westberg Hal , Geron Christopher , Guenther Alex , Klinger Lee , Harley Peter , Zimmerman Pat
Published: February 2004

During the summer of 1992, isoprene emissions were measured in a mixed deciduous forest near Oak Ridge, Tennessee. Measurements were aimed at the experimental scale‐up of emissions from the leaf level to the forest canopy to the mixed layer. Results from the scale‐up study are compared to different canopy models for determining the leaf microclimate as input to isoprene emission algorithms. These include (1) no canopy effects, (2) a simple vertical scaling canopy model with a leaf energy balance, and (3) a numerical canopy model which accounts for leaf‐sun geometries, photosynthesis, respiration, transpiration, and gas transport in the canopy. Initial evaluation of the models was based upon a standard emission rate factor of 90 μgC g−1 hr−1 (0.42 nmol g−1 s−1) taken from leaf cuvette measurements and a biomass density factor of 203 g m−2 taken from biomass surveys and a flux footprint analysis. The results indicated that predicted fluxes were consistent among the models to within approximately ±20%, but that the models overestimated the mean flux by about a factor of 2 and overestimated the maximum observed flux by 30 to 50%. Adjusting the standard emission factor and biomass density each downward by 20% yielded predicted means approximately 20% greater than the observed means and predicted maxima approximately 25% less than the observed maxima. Accounting for changes in biomass density as a function of direction upwind of the tower improved the overall model performance.
The Global Energy and Water Cycle Experiment (GEWEX) Continental‐Scale International Project (GCIP): An overview
Authors: Coughlan Michael , Avissar Roni
Published: February 2004

This paper provides a brief historical overview of the Global Energy and Water Cycle Experiment (GEWEX) Continental‐Scale International Project (GCIP), the basis for selecting the principal site for this experiment, the scientific objectives, and the strategic planning that has been brought to bear in preparing for the implementation of the project. It also provides a summary of the 25 papers published in this special issue, which collectively represent the launching point for research in the GEWEX's initiatives on continental‐scale hydrometeorology.
A comparison of aircraft and ground‐based measurements at Mauna Loa Observatory, Hawaii, during GTE PEM‐West and MLOPEX 2
Authors: Atlas E. , Ridley B. , Walega J. , Greenberg J. , Kok G. , Staffelbach T. , Schauffler S. , Lind J. , Hübler G. , Norton R. , Dlugokencky E. , Elkins J. , Oltmans S. , Mackay G. , Karecki D.
Published: February 2004

During October 19–20, 1991, one flight of the NASA Global Tropospheric Experiment (GTE) Pacific Exploratory Mission (PEM‐West A) mission was conducted near Hawaii as an intercomparison with ground‐based measurements of the Mauna Loa Observatory Photochemistry Experiment (MLOPEX 2) and the NOAA Climate Modeling and Diagnostics Laboratory (CMDL). Ozone, reactive nitrogen species, peroxides, hydrocarbons, and halogenated hydrocarbons were measured by investigators aboard the DC‐8 aircraft and at the ground site. Lidar cross sections of ozone revealed a complex air mass structure near the island of Hawaii which was evidenced by large variation in some trace gas mixing ratios. This variation limited the time and spatial scales for direct measurement intercomparisons. Where differences occurred between measurements in the same air masses, the intercomparison suggested that biases for some trace gases was due to different calibration scales or, in some cases, instrumental or sampling biases. Relatively large uncertainties were associated with those trace gases present in the low parts per trillion by volume range. Trace gas correlations were used to expand the scope of the intercomparison to identify consistent trends between the different data sets.
Global carbon exchange and methane emissions from natural wetlands: Application of a process‐based model
Authors: Cao Mingkui , Marshall Stewart , Gregson Keith
Published: February 2004

Wetlands are one of the most important sources of atmospheric methane (CH4), but the strength of this source is still highly uncertain. To improve estimates of CH4 emission at the regional and global scales and predict future variation requires a process‐based model integrating the controls of climatic and edaphic factors and complex biological processes over CH4 flux rates. This study used a methane emission model based on the hypothesis that plant primary production and soil organic matter decomposition act to control the supply of substrate needed by methanogens; the rate of substrate supply and environmental factors, in turn, control the rate of CH4 production, and the balance between CH4 production and methanotrophic oxidation determines the rate of CH4 emission into the atmosphere. Coupled to data sets for climate, vegetation, soil, and wetland distribution, the model was used to calculate spatial and seasonal distributions of CH4 emissions at a resolution of 1° latitude × 1° longitude. The calculated net primary production (NPP) of wetlands ranged from 45 g C m−2 yr−1 for northern bogs to 820 g C m−2 yr−1 for tropical swamps. CH4 emission rates from individual gridcells ranged from 0.0 to 661 mg CH4 m−2 d−1, with a mean of 40 mg CH4 m−2 d−1 for northern wetland, 150 mg CH4 m−2 d−1 for temperate wetland, and 199 mg CH4 m−2 d−1 for tropical wetland. Total CH4 emission was 92 Tg yr−1. Sensitivity analysis showed that the response of CH4 emission to climate change depends upon the combined effects of soil carbon storage, rate of decomposition, soil moisture and activity of methanogens.