bluuebunny/crossref_metadata_2025 huggingface dataset:
Simulations of CFC content and water mass age in the deep North Atlantic
Authors: England Matthew H. , Holloway Greg
Published: February 2004

A series of numerical model experiments are studied with a view to determine whether the chlorofluorocarbon (CFC) burden in the North Atlantic Deep Western Boundary Current (DWBC) can be simulated realistically in models of coarse horizontal resolution. Five main model cases are considered, covering three different parameterizations of subgrid‐scale mixing (standard Cartesian, isopycnal, and the eddy advection scheme of Gent et al. [1995]), a version with enhanced surface thermohaline forcing, and, finally, a case that incorporates the effects of topographic stress as in Alvarez et al. [1994]. Most model experiments exhibit deficient CFC content in the DWBC, except, notably, the case with a tightly constrained wintertime thermohaline forcing in the North Atlantic, as well as the run which includes the effects of topographic stress. However, the approach of strongly restoring surface temperature‐salinity to observations yields excessive heat loss and large freshwater fluxes during winter and so cannot be recommended as a technique to rectify deficient North Atlantic Deep Water production in ocean models. Overall, the coarse‐resolution simulations only resolve a relatively weak and diffuse CFC signal in the DWBC in the North Atlantic and typically only capture one CFC‐bearing core. This is primarily due to model currents in the deep North Atlantic being too slow and broad compared to the real ocean, as well as a spurious tendency for the outflow to recirculate and upwell into the Gulf Stream. The model runs do not capture a deeper CFC core because they all fail to simulate a dense water overflow off the Greenland‐Iceland‐Scotland Ridge. Seawater age is analysed in each of the experiments using an idealized age tracer as well as the simulated CFC‐11/CFC‐12 ratio. The CFC ratio estimates water in the DWBC at the equator to be between 25 and 33 years old. In contrast, the equilibrated age tracer ranges from 141 to 210 years. The CFC‐estimated age is substantially younger than the true seawater age because of a mixing bias when CFC‐free waters are entrained into the DWBC. This is possible because old CFC‐free waters cannot alter the CFC‐11/CFC‐12 ratio of CFC‐burdened water, even though the true age is affected by such a dilution. It is therefore suggested that caution be taken when adopting the CFC‐11/CFC‐12 ratio to extrapolate seawater age in waters where significant dilution with CFC‐free waters has occurred.
Estimation of wind stress using dual‐frequency TOPEX data
Authors: Elfouhaily Tanos , Vandemark Douglas , Gourrion Jérôme , Chapron Bertrand
Published: February 2004

The TOPEX/POSEIDON satellite carries the first dual‐frequency radar altimeter. Monofrequency (Ku‐band) algorithms are presently used to retrieve surface wind speed from the altimeter's radar cross‐section measurement (σ0Ku). These algorithms work reasonably well, but it is also known that altimeter wind estimates can be contaminated by residual effects, such as sea state, embedded in the σ0Ku measurement. Investigating the potential benefit of using two frequencies for wind retrieval, it is shown that a simple evaluation of TOPEX data yields previously unavailable information, particularly for high and low wind speeds. As the wind speed increases, the dual‐frequency data provides a measurement more directly linked to the short‐scale surface roughness, which in turn is associated with the local surface wind stress. Using a global TOPEX σ0° data set and TOPEX's significant wave height (Hs) estimate as a surrogate for the sea state's degree of development, it is also shown that differences between the two TOPEX σ0 measurements strongly evidence nonlocal sea state signature. A composite scattering theory is used to show how the dual‐frequency data can provide an improved friction velocity model, especially for winds above 7 m/s. A wind speed conversion is included using a sea state dependent drag coefficient fed with TOPEX Hs data. Two colocated TOPEX‐buoy data sets (from the National Data Buoy Center (NDBC) and the Structure des Echanges Mer‐Atmosphre, Proprietes des Heterogeneites Oceaniques: Recherche Expérimentale (SEMAPHORE) campaign) are employed to test the new wind speed algorithm. A measurable improvement in wind speed estimation is obtained when compared to the monofrequency Witter and Chelton [1991] model.
Circulation, dense water formation, and outflow on the northeast Chukchi Shelf
Authors: Weingartner Thomas J. , Cavalieri Donald J. , Aagaard Knut , Sasaki Yasunori
Published: February 2004

We investigated circulation and water mass modification processes in the Chukchi Sea using (1) temperature, salinity, and velocity data collected between September 1991 and September 1992 from moorings in Bering Strait and the northeast shelf, and (2) meteorological data and ice concentrations derived from special sensor microwave imager (SSM/I) imagery. In October 1991 and from February to August 1992 the mean monthly circulation was steady and northward. From November to January, strong northeast winds diverted the low‐salinity Bering Inflow onto the western shelf, and weakened and reversed flow over the northeast shelf. The winds also opened extensive polynyas, wherein cold hypersaline (salinity > ∼34) waters formed. The fall/winter circulation enhanced dense water formation by diverting low‐salinity waters away from, and prolonging the residence time of water within, the polynyas. Alongshore convergence of the coastal flow swept some of the cold hypersaline water offshore (across isobaths), but the coastal current carried most of it into the Arctic Ocean through Barrow Canyon. The dense outflow mixed little during its descent through the upper canyon, where bottom friction and rotation, but not entrainment, were important in the plume momentum balance. If the outflow is not diluted by mixing in the lower canyon (where theory suggests this will occur), then the densest shelf waters can ventilate layers deeper than the halocline. Salt rejection estimates using SSM/I data and surface heat budget calculations agree, within a factor of 2, with independent estimates from the mooring. Hence remote sensing techniques can monitor winter salt rejection rates in the Chukchi Sea.
Transient tracers and air‐sea gas transfer
Authors: Asher William , Wanninkhof Rik
Published: February 2004

This paper provides a review of the physics and chemistry associated with air‐sea gas transfer of transient atmospheric trace gases and the available laboratory and field measurement techniques used to study air‐water gas transfer. The mechanistic principles and their relation to the measurement techniques are used to show that the error associated with estimating air‐sea transfer velocities of transient tracers from transfer velocities measured using proxy tracers can be significant if an incorrect dependence of the transfer velocity on molecular diffusivity is assumed. Bubble‐mediated transfer processes are also demonstrated to have a significant effect on the parameterization of the transfer velocity.
Modulation of the seasonal signal of the Kuroshio Extension during 1994 from satellite data
Authors: Adamec David
Published: February 2004

Interannual modulation of the seasonal signal in the Kuroshio Extension region is studied using TOPEX altimeter and advanced very high resolution radiometer (AVHRR) data. Meridional profiles of the surface eddy kinetic energy indicate a significant drop in eddy energy levels beginning early in 1994 and persisting until the summer of 1995. The low eddy energy levels are most apparent during the summer and fall 1994. The contribution of the convergence of Reynolds stresses to the zonal momentum budget,
, is much weaker in 1994 than in either 1993 or 1995, especially during summer. The low values of Reynolds stresses lead the low eddy kinetic energy values by about a season, consistent with the characteristic timescale of the eddy energy, which is about 90–110 days in this region. AVHRR‐derived estimates of sea surface temperature indicate that the subpolar gyre was cooler during summer 1994 relative to the summers of 1993 and 1995. Calculations of the baroclinic zonally symmetric circulation indicate that the Kuroshio Extension was frontolytic during summer 1994, which could account for the observed drop in eddy kinetic energy. In particular, convergence of heat due to eddy flow appears to play a crucial role in the interannual modulation of the seasonal signal of the Kuroshio Extension.
A model of the thermal bar circulation in a long basin
Authors: Gbah Messon B. , Jacobs Stanley J. , Meadows Guy A. , Bratkovich Alan
Published: February 2004

A thermal bar is defined here as a zone of descending water at or near the fresh water temperature of maximum density. In the present study, we model physical properties such as velocity, temperature, turbulent fluxes, time of onset, and migration speed of the thermal bar in an idealized basin. Unlike models using prescribed eddy viscosity and diffusivity coefficients, our model uses a second‐order turbulence closure scheme to model mixing due to turbulence. The only inputs in the calculation are the surface wind, the surface heat flux, and the initial state of the fluid. Numerical simulations of the resulting circulation are presented for flow in a long rotating basin under prescribed surface heat and wind conditions. The results show the influence of the relative magnitudes of mechanical and thermal forcing on the circulation. A comparison with field observations shows reasonable agreement between computed and measured bar migration speeds. The time necessary for the formation of the bar is computed, and is found to depend on the surface forcing.
The mean and variability of ocean circulation past northern New Zealand: Determining the representativeness of hydrographic climatologies
Authors: Roemmich Dean , Sutton Philip
Published: February 2004

Eastward flow in the Tasman Sea, from the separated East Australia Current, reattaches to the shelf break near North Cape, New Zealand, and then continues alongshore to the southeast as the East Auckland Current. A series of three permanent warm core eddies occurs along the offshore side. The mean transport of the East Auckland Current is about 9 Sv, with an additional 10 Sv or more of circulation in the eddies. An extensive hydrographic data set, archived broad scale expendable bathythermograph (XBT) data, two repeating high‐resolution XBT transects, neutrally buoyant float trajectories, and TOPEX altimetric data are used to estimate the temperature and absolute flow fields and to characterize variability. The aim is to examine the usefulness of time series information and absolute velocity measurements in the interpretation of hydrographic snapshots and climatologies, as well as to describe a region having intrinsic oceanographic interest and complexity. Issues of representativeness of the hydrographic data, of the magnitude and scales of the underlying variability, of the existence of permanent fine‐scale features, and of the appropriateness of deep reference levels are addressed directly. The relatively well sampled hydrographic climatology is shown to contain the equivalent of as many as 10 independent realizations. Temperature errors, relative to the true mean, are typically a few tenths of a degree. Significant seasonal bias is identified in the surface layer, and interannual bias is seen in the position of an eddy near North Cape. The dynamic height field at 1000 dbar relative to 2000 dbar is similar to estimates based on float trajectories and the assumption of geostrophic dynamics. This study underlines the value of time series data in the interpretation of a hydrographic climatology, in quantifying the errors in the estimated mean field as well as determining the magnitude and nature of variability. It also highlights the fact that the mean circulation of the oceans contains significant mesoscale structure, unnoticed in coarsely smoothed climatologies.
Eddy formation in the Denmark Strait overflow
Authors: Krauss Wolfgang , Käse Rolf H.
Published: February 2004

On the basis of observations along the continental slope of east Greenland, a primitive equation, open boundary model is used to study the formation of eddies in the Denmark Strait overflow. The eddies are produced by vortex tube stretching, initiated by cross‐slope components of the velocity field. A sequence of cyclones and anticyclones is formed; the cyclones are located along the deeper part of the slope, and the anticyclones are along the shallower slope. The eddies are almost barotropic with horizontal and vertical velocities of 30 cm s−1 and 0.1 cm s−1, respectively. The cyclones migrate along the isobaths with a slight downhill component and intensify on their path until they become influenced by the flat, deep basin. Their translation speed is typically 20–30 cm s−1, and the local change in relative vorticity is nearly entirely determined by vortex tube stretching and vorticity advection. The overflow water flows mainly between the cyclonic and anticyclonic eddy chains. The overflow is strongly accelerated by the eddies. The existence of anticyclonic eddies along the shallower part of the east Greenland continental slope, predicted by the numerical model, has been confirmed by observations.
Estimate of bottom and surface stress during a spring‐neap tide cycle by dynamical assimilation of tide gauge observations in the Chesapeake Bay
Authors: Spitz Y. H. , Klinck J. M.
Published: February 2004

Dynamical assimilation of surface elevation from tide gauges is investigated to estimate the bottom drag coefficient and surface stress as a first step in improving modeled tidal and wind‐driven circulation in the Chesapeake Bay. A two‐dimensional shallow water model and an adjoint variational method with a limited memory quasi‐Newton optimization algorithm are used to achieve this goal. Assimilation of tide gauge observations from 10 permanent stations in the Bay and use of a two‐dimensional model adequately estimate the bottom drag coefficient, wind stress, and surface elevation at the Bay mouth. Subsequent use of these estimates in the circulation model considerably improves the modeled surface elevation in the entire Bay. Assimilation of predicted tidal elevations yields a drag coefficient, defined in the hydraulic way, varying between 2.5×10−4 and 3.1×10−3. The bottom drag coefficient displays a periodicity corresponding to the spring‐neap tide cycle with a maximum value during neap tide and a minimum value during spring tide. From assimilation of actual tide gauge observations, it is found that the fortnightly modulation is altered during frontal passage. Furthermore, the response of the sea surface to the wind forcing is found to be more important in the lower Bay than in the upper Bay, where the barometric pressure effect seems to be more important.
Fracture of multiyear sea ice
Authors: Sammonds P. R. , Murrell S. A. F. , Rist M. A.
Published: February 2004

The fracture and flow of multiyear sea ice was investigated under triaxial compression and uniaxial tension in the temperature range −40° to −3.5°C, for strain rates from 10−7 to 10−2s−1 and for confining pressures up to 30 MPa using 40 mm diameter specimens. Specimens both in the horizontal plane of the multiyear floe and perpendicular to this plane were tested. The results of short‐rod fracture toughness tests on multiyear and first‐year sea ice at temperatures −20°C are also reported. The multiyear sea ice came from an unridged portion of a single floe about 7 m thick, which was found to be massive and not blocky with large voids. The ice had low salinity and high porosity. The inelastic deformation of multiyear sea ice was found to be strongly dependent upon strain rate, temperature, and confining pressure. In compression, four main types of deformation were observed. (1) Under uniaxial compression, completely brittle fracture at high strain rates (of 10−3 to 10−2 s−1) was characterized by multiple axial splitting. (2) Application of even a small confining pressure inhibited splitting, and fracture took place by the formation of a narrow shear fault inclined at 45±3°. (3) At higher confining pressures, plastic deformation accompanied substantial cracking activity. (4) However, at still higher confining pressures, cracking was completely inhibited and deformation was entirely plastic. At −20°C, shear fracture occurred according to a maximum shear stress criterion and hence was pressure independent, with crack nucleation dominating the fracture behavior. At −40°C, however, the shear fracture stress was found to be strongly pressure dependent up to 14 MPa and could be described in terms of a Coulombic failure envelope. The unusual 45° orientation of ice shear fractures, together with the unusual pressure dependencies of ice peak strengths, may be explained by the fact that low‐stress slip and cleavage occurs in the basal planes of ice crystals.
Observations of brine drainage networks and microstructure of first‐year sea ice
Authors: Cole D. M. , Shapiro L. H.
Published: February 2004

Brine drainage networks and the microstructure of first‐year sea ice have been examined at two locations near Barrow, northern Alaska. A method for obtaining full‐depth sections of ice sheets up to 1.8 m thick is presented and shown to provide information on the spatial distribution and geometry of brine drainage networks on a scale of meters. A number of such sections from the two test sites are presented which reveal a greater variety of main channel and side branch configurations than is typically observed in ice grown in the laboratory. Vertical and horizontal micrographs and thin section photographs were obtained in November 1993, and March and May 1994 at a test site in the relatively protected Elson Lagoon. The resulting time series of photographic records provide detailed information on the size, shape, and spatial distribution of the brine‐ and gas‐filled inclusions and a means to quantify their size and shape changes with time. An example of the changes with time in inclusion sizes and aspect ratios in the vertical and horizontal directions for a depth of 0.2 m, with a given thermal history is also presented.
Nucleation of feather cracks in columnar freshwater ice: Experimental observations
Authors: Gupta V. , Picu R. C.
Published: February 2004

Biaxial compression experiments were carried out to investigate the mechanism of feather crack nucleation in columnar freshwater ice. The tests were performed with a strain rate of 10−3 s−1 at −10°C. It is shown that the nucleation of feather cracks corresponds to a transition when the last of the remaining ligaments bonding the two grains fractures and the grain boundary transforms into a traction‐transmitting frictional crack. This is in contrast to the nucleation of wing cracks, proposed earlier, where the nucleation was due to the onset of viscous sliding of grains. In addition to the mechanism, the nucleation stress for feather cracks was measured for different confinement ratios. A fracture‐mechanics‐based model for the nucleation process is presented by V. Gupta and R. C. Picu [Nucleation of feather cracks in columnar freshwater ice: Theoretical modeling, manuscript in preparation].
Turbulence transport, vorticity dynamics, and solute mixing under plunging breaking waves in surf zone
Authors: Lin Pengzhi , Liu Philip L.‐F.
Published: February 2004

Plunging breaking waves generate turbulence and vorticity, which are of great importance for the solute and sediment transport in surf zone. In this paper the complex breaking processes are simulated by using an accurate numerical model that solves the Reynolds equations for the mean flow and modified k‐ε equations for the turbulence field. A solute transport model is employed to investigate the solute mixing under plunging waves. After validation of the numerical model by comparing numerical results with available experimental data, the numerical model is further utilized to study the detailed mechanisms of turbulence transport and vorticity dynamics. The differences between spilling and plunging breaking waves are discussed. The impact of the wave breaking on solute mixing in the surf zone is also examined.
Two‐layer model simulations using observation and model‐based wind stresses of the 1985–1992 thermocline depth anomalies in the tropical Pacific
Authors: Sirven J. , Frankignoul C. , Février S. , Sennéchael N. , Bonjean F.
Published: February 2004

A multivariate model testing procedure is used to investigate the thermocline depth anomalies in the tropical Pacific during 1985–1992 simulated by a two‐layer model forced by five different surface wind stress fields. Two of these are derived from the pseudo wind stress produced from surface marine reports at the Florida State University (FSU), using a constant and a stability dependent drag coefficient. The others come from the response of three atmospheric general circulation models (ARPEGE, ECHAM3, and Hadley Centre) to the observed sea surface temperature. The simulations are compared to a new bimonthly analysis of the 20°C isotherm depth anomalies based on ship and moored buoy measurements. The best model‐data fit is gained when the forcing uncertainties are taken into account, considering the multiple simulations as representative of the latter. But they are still insufficient to explain the model‐reality discrepancies. However, although the two‐layer model has serious limitations in the eastern Pacific where the simulated oceanic response is minimal wrhile the observed variability is large, the testing procedure shows that it reproduces the main features of the observed variability and may thus be used to understand how, in a simulation, a particular feature of a forcing field translates. It is found that the simulations based on the FSU product and the Hadley Centre wind stresses are comparable in realism, and are significantly more realistic than the ones based on the ARPEGE or ECHAM3 wind stresses, because these misrepresent the phase of the 1986–1987 El Niño event. In addition, the ECHAM3 wind stress peaks off the equator, decreasing the quality of the two‐layer model response.
Biological and chemical signs of upward motions in permanent geostrophic fronts of the western Mediterranean
Authors: Zakardjian Bruno , Prieur Louis
Published: February 2004

Upward motions are often invoked to explain the high productivity of permanent geostrophic fronts in the Western Mediterranean while physical evidence of such upward advections is seldom reported. The goal of this study is to define biological and chemical criteria, which can be used to localize such upward motions zones. We use a one‐dimensional, time‐dependent model of phytoplankton dynamics to test the effects of upward advection on the vertical distribution of phytoplankton biomass, nutrients, and dissolved oxygen. Simulations also include the effects of advective motions of the phytoplankton cells in the light field on phytoplankton growth. In conformance with the continuity equation, boundary conditions were defined to allow horizontal flow of the upwelled water within the upper mixed layer. Low upward advections (≤3 m d−1) led to a shallowing and sharpening of the nitracline, oxycline, and deep maxima of phytoplankton biomass and oxygen and to an increase in phytoplankton biomass. By confining the phytoplankton‐nutrient system in the surface mixed layer, higher upward advections lead to homogeneous phytoplankton biomass and oxygen vertical distributions in the upper mixed layer, the nitracline and the oxycline being then at the top of the pycnocline. Data collected during the Prolig 2 cruise (May 1985) on the heavy side of the Liguro‐Provençal front are interpreted as an illustration of these numerical results. Computed primary production rates are compared with measurements conducted in the Almería‐Oran front during the Almofront 1 cruise (April 1991) in a similar situation. In both fronts, upward advections of 1–2 m d−1 would be sufficient to account for the observed vertical distributions and the increased primary production. Ecological implications for the phytoplankton‐nutrient system are discussed, particularly the spatial uncoupling of phytoplankton biomass and primary production in permanent geostrophic fronts.
Formation of North Pacific Intermediate Water in Meteorological Research Institute ocean general circulation model: 1. Subgrid‐scale mixing and marginal sea fresh water
Authors: Yamanaka Goro , Kitamura Yoshiteru , Endoh Masahiro
Published: February 2004

Formation of North Pacific Intermediate Water (NPIW) is investigated using an ocean general circulation model for the North Pacific with realistic bottom topography. The model has a horizontal resolution of 2° longitude by 1° latitude and 28 vertical levels. Experiments explored the sensitivity of the NPIW property to (1) subgrid‐scale mixing parameterization, (2) surface flux conditions for temperature and salinity, and (3) fresh water sources in marginal seas. A combination of horizontal mixing and sea surface temperature and salinity relaxation toward annual mean climatological values simulates the salinity minimum of the modeled NPIW very poorly. Inclusion of isopycnal subgridscale mixing substantially improves the salinity structure. A low‐salinity tongue associated with the salinity minimum, for example, extends from the subarctic gyre to the subtropical gyre at intermediate depths, while the salinity minimum in the modeled NPIW appears at 26.0–26.2 σθ, compared to 26.8 σθ for the observed NPIW. When surface condition for temperature and salinity restored to perpetual winter is imposed, the isopycnal surface of 26.0–26.2 σθ outcrops in the northwestern subtropical gyre, and salinity increases effectively due to excessive evaporation over precipitation. This allows the NPIW density to increase to 26.4–26.6 σθ. When temperature and salinity are restored to climatological values from the surface to the bottom in the Okhotsk and Bering Seas, less saline water flowing from marginal seas sets the NPIW density to the observed 26.8 σθ. Spatial distribution of modeled NPIW is found sensitive to the isopycnal diffusion. If the isopycnal diffusion is reduced, a low‐salinity tongue of the modeled NPIW disappears in the western North Pacific; relatively large isopycnal diffusion is necessary to simulate a fresh NPIW tongue in this area. Experiments with and without water flux from the northern marginal seas to the North Pacific confirm that the modeled NPIW has two fresh water sources. It is found that the northwestern subarctic sea surface supplies fresh water to the upper NPIW (26.4–26.7 σθ), whereas the Okhotsk Sea supplies fresh water to the lower NPIW (26.8–27.4 σθ).
Mixing of a tracer in the pycnocline
Authors: Ledwell James R. , Watson Andrew J. , Law Clifford S.
Published: February 2004

A patch of sulfur hexafluoride was released in May 1992 in the eastern North Atlantic on an isopycnal surface near 300 m depth and was surveyed over a period of 30 months as it dispersed across and along isopycnal surfaces. The diapycnal eddy diffusivity K estimated for the first 6 months was 0.12±0.02 cm2/s, while for the subsequent 24 months it was 0.17±0.02 cm2/s. The vertical tracer distribution remained very close to Gaussian for the full 30 months, as the root mean square (rms) dispersion grew from 5 to 50 m. Lateral dispersion was measured on several scales. The growth of the rms width of the tracer streaks from less than 100 m to approximately 300 m within 2 weeks implies an isopycnal diffusivity of 0.07 m2/s at scales of 0.1 to 1 km, larger than expected from the interaction between vertical shear of the internal waves and diapycnal mixing. Teasing of the overall patch, initially about 25 km across, into streaks with an overall length of 1800 km within 6 months supports predictions of exponential growth by the mesoscale strain field at a rate of 3±0.5 × 10−7 s−1. The rms width of these streaks, estimated as 3 km and maintained in the face of the streak growth, indicates an isopycnal diffusivity of 2 m2/s at scales of 1 to 10 km, much greater than expected from internal wave shear dispersion. The patch was painted in, albeit streakily, by 12 months, confirming expectations from analytical and numerical models. Homogenization of the patch continued during the subsequent 18 months, while the patch continued to spread with an effective isopycnal eddy diffusivity on the order of 1000 m2/s, acting at scales of 30 to 300 km.
Preliminary results from directly measuring middepth circulation in the tropical and South Pacific
Authors: Davis Russ E.
Published: February 2004

As part of the World Ocean Circulation Experiment, 300 Autonomous Lagrangian Circulation Explorer floats were deployed in the tropical and South Pacific to measure the general circulation at 900 m depth. Most floats measured average currents over 25 submerged days between ascents to the surface for locating and data relay. By the end of 1997, over 12,500 observations spanning 840 float years were obtained. Accuracy of the average velocity is limited by variability, some of which is found to arise from steady flows with spatial variations smaller than averaging scales. Observed 25‐day velocities are serially uncorrelated except near the equator where energetic zonal variability has seasonal timescales and appears to be described by a data‐assimilating ocean circulation model run by the National Centers for Environmental Prediction. Space‐time averages of float velocities disclose a general pattern of middepth flow that resembles the surface circulation with the strongest flows of O (3 cm/s) in the deep Antarctic Circumpolar Current and East Australian Current. The weaker elements of the flow, including an interior subtropical circulation with O (millimeters per second) flows, are described by an objective analysis whose accuracy is limited by sampling noise rather than uncertainty of the analysis parameters. The subtropical circulation has a gyres‐within‐a‐gyre structure including boundary currents along Australia and east of New Zealand and a concentrated equatorial limb with surprising strength in the western basin. This current is also involved in a tropical gyre with generally eastward flow along the equator in the western basin at least. Because of strong seasonal‐scale variability, the equatorial zonal currents are not well measured yet but data are still being accumulated. Comparison with Reid's [1997] absolute circulation shows more areas of agreement than of disagreement.
Mean circulation and transports in the South Atlantic Ocean: Combining model and drifter data
Authors: Stutzer S. , Krauss W.
Published: February 2004

Numerical experiments with a medium‐resolution primitive equation model of the South Atlantic mean circulation are described. The results from the standard model realization indicate that the model succeeds in representing the large‐scale transport and circulation features. However, a comparison with a velocity field derived from surface drifter data reveals discrepancies of the modeled velocities from the observations in magnitude as well as direction of the flow field. In order to diminish the model deviations from the data, an attempt is made to couple the model to the observations through a simple data assimilation technique. The assimilated model succeeds in improving the subtropical gyre circulation. Only a minor effect on the basin‐scale integrated quantities is observed. However, the density field may be deformed as a response to the assimilation of velocity data without simultaneously adapting a corresponding density structure. The influence of the disturbance of the density structure is most prominent at the edges of the observed data set, which does not cover the entire model domain, and is confined to the upper ocean and balanced above the thermocline. We calculated a meridional heat transport that is generally in accordance with estimates from other sources. The analysis of heat and salt fluxes suggests that the model features both the so‐called “warm water path” and “cold water path” in closing the global thermohaline circulation. While heat is mainly imported in surface and thermocline waters with the Agulhas Current around South Africa, it is the Antarctic Intermediate Water that compensates for more than 50% of the salt loss by the outflowing North Atlantic Deep Water.
An airborne expendable bathythermograph survey of the South China Sea, May 1995
Authors: Chu Peter C. , Fan Chenwu , Lozano Carlos J. , Kerling Jeffrey L.
Published: February 2004

An extensive airborne expendable bathythermograph survey of the South China Sea (SCS) conducted in May 1995 and historical data are used to analyze and infer the upper layer (300 m) synoptic structure and general circulation. The primary thermal feature observed was a central SCS warm pool surrounded by several cool pools. The size of the warm pool decreased with depth from approximately 200,000 km2 at 50 m depth to about 70,000 km2 at 300 m depth. The maximum temperature of the warm pool was 30°C, appearing near the surface. At the depth of 50 m, the temperature of the central SCS warm pool was 29°C, and the temperature of the five surrounding cool pools ranged from 26°C to 22°C. A three‐dimensional estimate of the absolute velocity field was obtained from the observed temperature field and a climatological salinity field using the β spiral method. Striking circulation features were the existence of dual anticyclonic eddies in the central SCS warm pool and the existence of cyclonic eddies associated with the cool pools. In the upper layer the tangential velocity of the dual central SCS anticyclonic warm‐core eddies is around 30–40 cm/s and that of the five cyclonic cool‐core eddies varies from 10 cm/s to 40 cm/s. The tangential velocity of all the eddies decreased with depth. At 300 m depth, it became less than 5 cm/s for all the eddies.
Surface longwave radiation conditions in the eastern Weddell Sea during winter
Authors: Guest Peter S.
Published: February 2004

This is a study of the surface longwave radiation characteristics over the Weddell Sea during winter, based on measurements performed from a ship and ice floe camps during the Antarctic Zone Flux Experiment (ANZFLUX), which occurred in July and August 1994. Net longwave radiation is the dominant term in the regional surface energy balance. The net longwave radiational cooling of the surface is 62 W m−2 greater during clear skies than during overcast skies. The downwelling longwave radiation during clear skies is 17% less, and during overcast skies, it is 3% less than for reported Arctic cases with the same surface temperature. Previously published formulas of downwelling longwave radiation based on surface variables have a total rms error of 10–43 W m−2 for the clear‐sky cases and 10–20 W m−2 for overcast cases when applied to the ANZFLUX data. There is no correlation of downward longwave radiation with any standard surface meteorological parameters, except air temperature and observed cloud cover, with the possible exception of wind speed, which has a slight negative correlation during clear skies. Predictions from newly derived formulas using only air temperature have an rms error of 5.8 W m−2 for overcast skies and 7.2 W m−2 for clear skies. When information on cloud bottom temperature and cloud thickness is available, downwelling radiation is predicted to within 3.9 W m−2. These rms errors are based on parameterizations that are tuned to the ANZFLUX data set and are better than the accuracies to be expected when applying the parameterizations to other situations, including other Weddell Sea wintertime cases.
Seasonal variability of heat transport derived from hydrographic and wind stress data
Authors: Kobayashi Taiyo , Imasato Norihisa
Published: February 2004

Oceanic heat transport and its seasonal variations are determined with a diagnostically calculated seasonal velocity and an observed seasonal temperature. The heat transport of the Indonesian Throughflow varies greatly from 0.82 to 1.62 PW (1 PW = 1015 W). The northward heat transport through the South Atlantic is similar to the heat transport of the Agulhas Leakage, but the phases of these variations are opposite to each other. Hence the South Atlantic circulation alone produces a southward heat transport during boreal winter, and the heat from the Indian Ocean maintains a northward heat flux in the South Atlantic. The meridional heat transport in the global ocean varies with an almost annual cycle, and a large amount of heat is transported toward the winter pole (7.0 PW northward and 3.1 PW southward) in the tropical ocean by the surface Ekman current. Although the meridional heat transport has large variations, the nonlinear contribution to the annual mean is very small in the present model. The nonlinear heat transport is mainly produced by seasonal variations of the surface temperature and the Ekman current. Hence further studies with a fine grid model to resolve the seasonal variation of the western boundary currents will be required to properly address the nonlinearity of the heat transport.
Stable oxygen isotopic composition of corals from the Gulf of Guinea as indicators of periods of extreme precipitation conditions in the sub‐Sahara
Authors: Swart Peter K. , White Kathy S. , Enfield David , Dodge Richard E. , Milne Peter
Published: February 2004

Stable oxygen isotopic analyses of scleractinian coral skeletons from the Gulf of Guinea in the eastern Atlantic reveal that the corals from this region can be used to identify periods of severe drought and above average precipitation in sub‐Sahara Africa. Data presented in this paper show an inverse correlation between precipitation in the Sahel and the δ18O values of a coral skeletons of the species Siderastrea spp. collected from the island of Principe in the Gulf of Guinea. This is opposite to the correlation expected, as previous work has suggested that higher sea surface temperatures occur in the Gulf of Guinea during periods of low rainfall in the Sahel. Such an association would lead to a positive correlation between Sahel precipitation and skeletal δ18O. The explanation for the observed inverse correlation is that the salinity of the Gulf of Guinea is strongly influenced by the outflow from the Niger and Congo rivers. These periods of high freshwater input also correlate with periods of higher rainfall in the sub‐Sahara and therefore affect the δ18O values of the coral skeleton. The correlation between δ18O values of the coral skeleton and temperature in the northern subtropical Atlantic Ocean (r = −0.34), the magnitude of the dipole (r = −0.45), and the latitudinal position of the intertropical convergence zone (r = −0.37) illustrate that the δ18O values in the coral skeleton reflect climate dynamics of the region that affect the precipitation patterns in sub‐Sahara Africa.
Air‐sea interaction over a thermal marine front in the Denmark Strait
Authors: Vihma Timo , Uotila Juha , Launiainen Jouko
Published: February 2004

An investigation was conducted into air‐sea interaction in the Denmark Strait, where a distinct thermal front separates warm North Atlantic water from the cold East Greenland Current. The field data consisted of ship weather station data and rawinsonde soundings from R/V Aranda's expedition in August‐September 1993. The surface energy balance differed drastically between the warm and cold side of the front (net fluxes of 95 W m−2 upward and 82 W m−2 downward, respectively). The difference resulted mostly from the contradictory turbulent fluxes. The air temperature, humidity and wind speed showed more variation on the warm side of the front. Lower wind speeds were observed on the cold side. The cross‐frontal differences in the air temperature and wind speed were largest during front‐parallel flow, but those in the sensible and latent heat flux were largest during cross‐frontal flow. During cases of air advection across the front, the modification in the air temperature was strongest with a low wind speed. Downwind of the front, the sensible heat flux strongly depended on the south‐north wind component. The rawinsonde data revealed temperature inversions and low‐level jets. The wind profile was affected by the combined effects of baroclinity, surface layer stability, and stratification through the atmospheric boundary layer. The surface heterogeneity caused by the sea surface temperature front resulted in the Schmidt paradox: the area‐averaged sensible heat flux was upward, while the area‐averaged air temperature exceeded the area‐averaged surface temperature. A mosaic method, extended by estimates of the local wind speed over the warm and cold water side, was applicable to parameterizing the area‐averaged sensible heat flux.
An interdecadal climate cycle in the South Atlantic and its links to other ocean basins
Authors: Venegas Silvia A. , Mysak Lawrence A. , Straub David N.
Published: February 2004

A singular value decomposition analysis and a combined complex empirical orthogonal function analysis are performed on 80 years of monthly sea surface temperature (SST) and sea level pressure (SLP) data from the South Atlantic region. The analyses reveal the existence of interdecadal fluctuations in the coupled atmosphere‐ocean system with a period of around 20 years. The SST anomalies are observed to propagate anticyclonically around the South Atlantic basin following the subtropical gyre circulation. At the same time, a westward propagation of SLP anomalies across the basin generates changes in the atmospheric circulation that appear to reinforce such SST anomalies through anomalous exchanges of heat. It is thus proposed that the dominant physical processes involved in this interdecadal cycle include the horizontal advection of heat by the ocean currents and changes in the atmosphere‐ocean heat fluxes through local air‐sea interactions. The global SST and SLP patterns that accompany the different phases of the South Atlantic cycle are also presented. They show similarities with other well‐known interdecadal signals observed by several investigators in other ocean basins (e.g., the low‐frequency part of the North Atlantic Oscillation). This suggests that the South Atlantic signal described in this study may be a regional aspect of global interdecadal variability.
Anthropogenic carbon dioxide in the Arctic Ocean: Inventory and sinks
Authors: Anderson Leif G. , Olsson Kristina , Jones E. Peter , Chierici Melissa , Fransson Agneta
Published: February 2004

An inventory and sequestering rate of anthropogenic carbon dioxide CTanthro in the Arctic Ocean, calculated by a plume‐entrainment model, are presented. The plume is initiated by a fraction rj leaving the shelf break at 200 m, followed by an entrainment of rj for every 150 m depth the plume descends. The model is constrained by the CFC‐12 and carbon tetrachloride (CCl4) distributions, with the concentrations of CFC‐12, CCl4, and CTanthro in the source water calculated assuming a water in 100% equilibrium with the atmosphere. The model is run from 1750 to 1991, the latter being the year in which measurements of the transient tracers in the water column of the central Arctic Ocean were made. The output from the model gives sinks of anthropogenic carbon dioxide in 1991 of 0.026±0.009 Gt C yr−1, of which 0.0194 Gt C yr−1 is in the Eurasian Basin and 0.0070 Gt C yr−1 in the Canadian Basin. This amounts to about 1% of the total oceanic uptake of anthropogenic CO2. The Arctic Ocean inventory of anthropogenic carbon dioxide in 1991 was 1.35(+0.12/−0.06) Gt C, which is about 1% of the total oceanic inventory. The sensitivity of the computed sinks and inventories to various model assumptions was estimated.
Swash infiltration‐exfiltration and sediment transport
Authors: Turner Ian L. , Masselink Gerhard
Published: February 2004

Field measurements of vertical pore‐pressure gradients within the bed are used to quantify instantaneous (8 Hz) rates of swash infiltration‐exfiltration across the beach face. Cyclic infiltration‐exfiltration is associated with individual swash events, with observed vertical flow rates O(10−3) m/s. Rates of net swash‐groundwater exchange (i.e., through‐bed flow integrated over several swash cycles) are two orders of magnitude smaller. At the timescale of individual swashes, vertical pore‐pressure gradients within the beach face are much greater than horizontal pore‐pressure gradients. This permits application of the numerical solution of Darcy's law for one‐dimensional vertical flow to model fluctuating pore pressures (and hence vertical through‐bed flow). Vertical flow through a porous bed modifies sediment mobility in (at least) two ways: (1) Seepage forces change the effective weight of surficial sediments, and (2) boundary layer “thickening” or “thinning” result in altered bed shear stresses. By considering these two (opposing) effects separately, a new Shields parameter is derived that incorporates terms for through‐bed flow. Simulation of time‐varying seepage force and bed stress effects over an uprush‐backwash cycle suggests that the effect of altered bed stresses dominates over the change in effective weight and that infiltration‐exfiltration effects are most important during uprush. Simulated transport rates are increased by up to 40% of the peak transport rate during uprush and reduced by 10% during backrush. In summary, swash infiltration‐exfiltration across a saturated beach face enhances the net upslope transport of sediment.
Vegetation detection through smoke‐filled AVIRIS images: An assessment using MODIS band passes
Authors: Miura T. , Huete A. R. , van Leeuwen W. J. D. , Didan K.
Published: February 2004

Radiometrically calibrated, Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) images acquired during the Smoke, Clouds and Radiation in Brazil (SCAR‐B) experiment were processed to simulate vegetation index (VI) imagery with the Moderate Resolution Imaging Spectroradiometer (MODIS) band passes. Data sets were extracted from tropical forested areas, burned fields, and shrub/grassland areas over both clear and variable smoke conditions with average aerosol optical thickness (AOT) values at 0.67 μm of 0.14, 1.1, and 1.9, respectively. The atmospheric resistant VIs and various middle‐infrared (MIR) derived VIs were then analyzed with respect to their ability to minimize atmospheric “smoke” contamination. The atmospheric resistant VIs utilized the blue band for correction of the red band, while the MIR‐derived VIs used the MIR region (1.3–2.5 μm) as a substitute for the red band since it is relatively transparent to smoke, yet remains sensitive to green vegetation. The performance of these indices were assessed and compared with the normalized difference vegetation index (NDVI) and the soil‐adjusted vegetation index (SAVI). Over the tropical forests the NDVI and SAVI had high relative errors over all smoke‐filled atmospheric conditions (50–80% error), while the atmospheric resistant VIs resulted in a 50–80% relative error only over thick levels of smoke. Over optically thin levels (AOT at 0.67 μm <1.1) they performed much better with a 20–40% relative error. The MIR‐derived VIs, on the other hand, outperformed all other VIs over forested areas (≤ 5% error). However, over burned fields with minimal amounts of green biomass the MIR‐derived VIs had the highest levels of error due to smoke (>40%), while all other indices had errors below 20%. In the shrub/grassland site, the atmospheric resistant indices behaved similarly with the MIR‐derived indices, with both less sensitive to smoke than the NDVI and SAVI. We conclude that the MIR indices, particularly with MODIS band 7 (2.13 μm), are useful in vegetation monitoring over forested areas during the burning season. However, they did not perform well in areas outside of forests such as burned areas and shrub/grassland.