bluuebunny/crossref_metadata_2025 huggingface dataset:
Characterization of the Antarctic Polar Frontal Zone to the north of South Georgia in summer 1994
Authors: Trathan P. N. , Brandon M. A. , Murphy E. J.
Published: February 2004

The Polar Front (PF) forms the southern boundary to the Polar Frontal Zone (PFZ) along the northern edge of the Antarctic Circumpolar Current (ACC). In a number of areas the position of the PF (and thus the PFZ) is known to be influenced by topographic steering, while local bathymetry has also been implicated in the movement and retention of various associated mesoscale features. In this paper we examine the structure and position of the PF as it passes over the rugged bathymetry to the north of the Scotia Sea. Results are presented from an oceanographic transect crossing the PF to the northwest of South Georgia and from a pair of shorter transects south of the PF but north and east of the first. Associated with the PF was a narrow, high‐speed flow embedded in broader, slower moving regions. This high‐speed flow was found to have a geostrophic component of velocity that was slower than estimates for other regions of the PF. Comparisons with output from recent oceanographic models were found to be consistent with the physical observations. A second examination of the region after a period of 30 days suggested that the surface expression of the PF had shifted southward by approximately 35 km but that the PF was essentially in the same position over the southern edge of the Maurice Ewing Bank. An advanced very high resolution radiometer image taken during the cruise provided additional information about the position of the surface expression of the PF and the extent of mesoscale features that were present to the north of the study area. Immediately to the north of South Georgia, water in the eastward flow of the ACC meets colder, more saline water flowing west along the north coast of the island. The area where these two flows meet was found to be variable over the 30‐day timescale of the cruise. This area is known to be of major biological significance, and variability in the local oceanography is possibly of crucial importance to many predator species breeding at the northern end of South Georgia.
Inertial dissipation method applied to derive turbulent fluxes over the ocean during the Surface of the Ocean, Fluxes and Interactions with the Atmosphere/Atlantic Stratocumulus Transition Experiment (SOFIA/ASTEX) and Structure des Echanges Mer‐Atmosphere, Proprietes des Heterogeneites Oceaniques: Recherche Experimentale (SEMAPHORE) experiments with low to moderate wind speeds
Authors: Dupuis Hélène , Taylor Peter K. , Weill Alain , Katsaros K.
Published: February 2004

The transfer coefficients for momentum and heat have been determined for 10 m neutral wind speeds (U10n) between 0 and 12 m/s using data from the Surface of the Ocean, Fluxes and Interactions with the Atmosphere (SOFIA) and Structure des Echanges Mer‐Atmosphere, Proprietes des Heterogeneites Oceaniques: Recherche Experimentale (SEMAPHORE) experiments. The inertial dissipation method was applied to wind and pseudo virtual temperature spectra from a sonic anemometer, mounted on a platform (ship) which was moving through the turbulence field. Under unstable conditions the assumptions concerning the turbulent kinetic energy (TKE) budget appeared incorrect. Using a bulk estimate for the stability parameter, Z/L (where Z is the height and L is the Obukhov length), this resulted in anomalously low drag coefficients compared to neutral conditions. Determining Z/L iteratively, a low rate of convergence was achieved. It was concluded that the divergence of the turbulent transport of TKE was not negligible under unstable conditions. By minimizing the dependence of the calculated neutral drag coefficient on stability, this term was estimated at about −0.65Z/L. The resulting turbulent fluxes were then in close agreement with other studies at moderate wind speed. The drag and exchange coefficients for low wind speeds were found to be Cen × 103 = 2.79U10n−1 + 0.66 (U10n < 5.2 m/s), Cen × 103 = Chn × 103 = 1.2 (U10n ≥ 5.2 m/s), and Cdn × 103 = 11.710n−2 + 0.668 (U10n < 5.5 m/s), which imply a rapid increase of the coefficient values as the wind decreased within the smooth flow regime. The frozen turbulence hypothesis and the assumptions of isotropy and an inertial subrange were found to remain valid at these low wind speeds for these shipboard measurements. Incorporation of a free convection parameterization had little effect.
A study on wind‐driven circulation in the subarctic North Pacific using TOPEX/POSEIDON altimeter data
Authors: Isoguchi Osamu , Kawamura Hiroshi , Kono Tokihiro
Published: February 2004

Time‐dependent wind‐driven circulation in the subarctic region in the North Pacific is investigated by using TOPEX/POSEIDON (T/P) altimeter data and European Centre for Medium‐Range Weather Forecasts (ECMWF) wind data for about 2 years. The first empirical orthogonal function (EOF) of the sea level anomaly (SLA) without the variation related to the steric height change (SLA*) represents a basin‐sized oscillation, which is associated with the spin‐up and spin‐down of the subarctic gyre. Moreover, the time series of the first EOF corresponds to the Oyashio current variation measured by the moored current meter at about 1250‐m depth. The first and third EOFs of the wind stress curl fields represent basin‐sized south‐north oscillations being associated with changes in intensity and the latitudinal position of the Aleutian low. Time series of both the dominant EOFs of the SLA* and the wind stress curl anomaly are correlated well, which suggests that dominant variation of the SLA* is mainly controlled by the wind stress curl fields represented by the first and third EOFs. The time series of the first EOF of the SLA* is in agreement with that of Sverdrup transport estimated by the wind stress curl near 40°N with a correlation of 0.70. Both standard deviations of the T/P SLA‐derived transports in the interior and the western boundary regions are consistent with that of the Sverdrup transport in the same order of magnitude. These results suggest that sea surface height variation in the subarctic North Pacific is approximately explained by the time‐dependent wind‐driven circulation.
Expected uncertainty in satellite‐derived estimates of the surface radiation budget at high latitudes
Authors: Key Jeffrey R. , Schweiger Axel J. , Stone Robert S.
Published: February 2004

An analysis of spatial and temporal variations of the polar radiation budget will undoubtedly require the use of multispectral satellite data. How well we can estimate the radiation balance depends on how well we can estimate the physical and microphysical properties of the surface and atmosphere that directly affect it, e.g., surface temperature and albedo, cloud droplet effective radius, cloud optical depth, cloud thickness, and cloud height. Here we examine our current ability to estimate the high‐latitude surface radiation budget using visible and thermal satellite data. The method for estimating radiative fluxes incorporates estimates of surface and atmospheric parameters, so the accuracy with which these can be retrieved from satellite data is first assessed. The effects of errors in the estimates of these parameters on the surface net radiation during summer and winter are quantified, and the relative sensitivity of the net radiation budget to errors in individual parameters is assessed. The combined uncertainty is then determined and examined in light of validation data in the Arctic. The results show upper and lower bounds for the uncertainties between 7.9 and 41 W m−2 for instantaneous retrievals of net radiation. By far, the largest portion of the uncertainty in net radiation is associated with errors in the retrieval of surface temperature and surface albedo. Although improvements in retrievals are desirable, currently available methods can provide surface net radiation in the Arctic with uncertainties similar to those of surface‐based climatologies.
On the unsteady separation/intrusion of the Gaspé Current and variability in Baie des Chaleurs: Modeling studies
Authors: Gan Jianping , Ingram R. Grant , Greatbatch Richard J.
Published: February 2004

A numerical model has been used to study separation/intrusion of an unsteady, baroclinic coastal jet, the Gaspé Current (GC) and its impact on dynamics and thermodynamics of the Baie des Chaleurs (BdC, Gulf St. Lawrence, Canada). The model has 2½ layers with primitive equation dynamics and an embedded bulk mixed layer (ML) model. It is forced with observed atmospheric fluxes, as well as the GC. The simulations show that the variability in the Baie des Chaleurs is controlled by the characteristics of the unsteady GC separation/intrusion. On the basis of the dynamic and vorticity balance analyses, it is found that the separation is related to the adverse pressure gradient force which is induced by the ageostrophic terms in the momentum equations and to local vorticity intensification due to the inertial effect in the flow. The simulations demonstrate characteristics of unsteady separation. The presence of separation in an accelerating current, as well as in a linear current, with strong deceleration when momentum advection is neglected, gives new insight into the mechanisms of unsteady boundary current separation. Unsteady separation or intrusion occurrence depends on the magnitude of the GC transport, as well as its rate and duration of deceleration or acceleration. The GC intrusion generates cyclonic circulation in the BdC. Prevailing westerly winds reduce the cyclonic circulation inside the bay but have little effect on separation/intrusion near the entrance. The GC either intrudes along the coastline (attachment) or after separation (reattachment). Increasing vertical shear of the GC and offshore movement of the GC axis reduces the tendency to separate. The finding of an asymmetric response of GC separation/intrusion to symmetric GC forcing explains the monthly mean features seen in observations taken in the bay.
Coral radiocarbon constraints on the source of the Indonesian throughflow
Authors: Moore M. D. , Schrag D. P. , Kashgarian M.
Published: February 2004

Radiocarbon variability in Porites spp. corals from Guam and the Makassar Strait (Indonesian Seaway) was used to identify the source waters contributing to the Indonesian throughflow. Time series with bimonthly resolution were constructed using accelerator mass spectrometry. The seasonal variability ranges from 15 to 60‰, with large interannual variability. Δ14C values from Indonesia and Guam have a nearly identical range. Annual mean Δ14C values from Indonesia are 50 to 60‰ higher than in corals from Canton in the South Equatorial Current [Druffel, 1987 ]. These observations support a year‐round North Pacific source for the Indonesian throughflow and imply negligible contribution by South Equatorial Current water. The large seasonality in Δ14C values from both sites emphasizes the dynamic behavior of radiocarbon in the surface ocean and suggests that Δ14C time series of similar resolution can help constrain seasonal and interannual changes in ocean circulation in the Pacific over the last several decades.
A model study of tidal and residual flow in Delaware Bay and River
Authors: Walters Roy A.
Published: February 2004

A numerical model, along with existing field data, is used in an exploratory study of the dynamics of Delaware Bay and River. A three‐dimensional hydrodynamic model has been constructed using a finite element discretization in space and a harmonic representation in time. These choices allow for adequate resolution of geometric and flow features but still allow an efficient numerical solution. The nodes in the horizontal are arranged into triangular elements, and the nodes in the vertical are transformed into a terrain‐following coordinate system. The nodes in the vertical are arranged logarithmically from the bottom so that the model resolves the bottom boundary layer. This paper focuses on the hydrodynamics rather than the salt dynamics. The model results are sensitive to stratification and to the details of the time dependence of the vertical eddy viscosity, Au. After consideration of several processes, it was found that the stratification effects arise from the tidal straining of the vertical density profile by the velocity. A harmonic expansion of Au indicates that the frictional effects of a time‐varying vertical viscosity are more pronounced on the secondary constituents (S2, N2, O1, K1) than the dominant constituent (M2). Thus the ratio of the amplitude of the secondary constituents to the amplitude of the primary constituent provides a sensitive measure for comparison with field data. This comparison places constraints on the variability of Au.
Two circulation regimes of the wind‐driven Arctic Ocean
Authors: Proshutinsky A. Y. , Johnson M. A.
Published: February 2004

The major goal of this paper is to demonstrate the existence in the Arctic Ocean of two regimes of wind‐forced circulation. We simulated the vertically averaged currents, sea level heights, and ice drift in the Arctic Ocean from 1946 to 1993 using a two‐dimensional, wind‐forced, barotropic model that includes frictional coupling between the ocean and ice. The model has a spatial resolution of 55.5 km and is driven by winds, river runoff, and an imposed but realistic sea level slope between the Pacific and the Atlantic Oceans. There is a good agreement between velocities from observed buoy motions and velocities of modeled ice drift even though the model lacks ocean baroclinicity and ice thermodynamics. The results indicate that wind‐driven motion in the central Arctic alternates between anticyclonic and cyclonic circulation, with each regime persisting for 5–7 years, based upon our analysis of the modeled sea level and ice motion. Anticyclonic wind‐driven motion in the central Arctic appeared during 1946–1952, 1958–1963, 1972–1979, and 1984–1988, and cyclonic motion appeared during 1953–1957, 1964–1971, 1980–1983, and 1989–1993. Shifts from one regime to another are forced by changes in the location and intensity of the Icelandic low and the Siberian high. The two regimes may help explain the significant, basin‐scale changes in the Arctic's temperature and salinity structure observed recently, the Great Salinity Anomaly, and the variability of ice conditions in the Arctic Ocean.
Dependence of apparent optical properties on solar altitude: Experimental results based on mooring data collected in the Sargasso Sea
Authors: Stramska Malgorzata , Frye Daniel
Published: February 2004

The in situ variability of the vertical attenuation coefficient for downward irradiance (Kd) and radiance reflectance (RL) in response to changing solar altitude has been investigated. Observations were made 18 times per day from a fixed position mooring deployed in the Sargasso Sea, The mooring included instrumentation to measure spectral downwelling irradiance (Ed) and spectral upwelled radiance (Lu) at 15‐ and 35‐m depths. Time series data used for our analysis represent conditions of low, almost constant concentration of chlorophyll‐like pigments in the water and sunny or partly cloudy sky conditions. During sunny days the dominant component of the observed diel variability of the apparent optical properties (AOPs) was caused by the variable solar altitude. The statistical analysis performed on the data indicates a significant correlation between Kd and the cosine of the zenith angle of the direct solar beam after refraction at the air/water boundary (μ). These changes were wavelength dependent; Kd at 412, 443, 490, 510, and 555 nm decreased by 18–30%, while Kd at 665 and 683 nm increased by more than 50% when solar altitude increased from about 17° to 60° (i.e., when μ increased from 0.69 to 0.93). The vertical attenuation coefficient for photosynthetically available radiation (KPAR) decreased by about 25%. The daily amplitudes of similar changes in RL at the blue/green light were much smaller, up to about 4 to 8% at 412, 443,490, and 510 nm and 11% at 555 nm. However, at 665 and 683 nm, RL changed by more than 100%. This large increase of RL and decrease of Kd at 665 and 683 nm with Sun altitude (with μ) can be explained by the fact that the light field at red wavelengths was strongly affected by inelastic processes such as Raman scattering and natural fluorescence. Remote‐sensing techniques rely on the interpretation of reflectance and upwelled radiance ratios. We observed up to 8% changes in reflectance ratios and up to 17% changes in upwelling radiance ratios during the day. These maximum changes were observed for the ratios including Lu or RL at 555 nm, which were affected by inelastic scattering of light. Although our data illustrate the dependence of AOPs on the Sun altitude on a daily timescale, similar relationships are expected to be of importance on a seasonal scale and with latitude.
Removal of methyl bromide in coastal seawater: Chemical and biological rates
Authors: King Daniel B. , Saltzman Eric S.
Published: February 2004

A stable isotope tracer technique was used to investigate the loss rate of methyl bromide in surface ocean waters. Unfiltered and 0.2 μm‐filtered or autoclaved aliquants of Biscayne Bay seawater samples were spiked with 13CH3Br at roughly 10–100 times ambient concentrations (50–800 pM) and incubated for 10–30 hours. The concentration of 13CH3Br was monitored using gas chromatography with isotope dilution mass spectrometry, with CD3Br as the isotope spike. Removal rates in unfiltered aliquants were significantly faster than in the 0.2 μm‐filtered or autoclaved aliquants, indicating that some of the loss of methyl bromide was associated with particulate matter. Filtration experiments indicate that the particulate material responsible for methyl bromide loss is between 0.2 and 1.2 μm in diameter, suggesting that bacteria are likely to be responsible. The particulate‐related removal of methyl bromide was inhibited by autoclaving, supporting a biological mechanism.
A model for microwave Doppler sea return at high incidence angles: Bragg scattering from bound, tilted waves
Authors: Plant William J.
Published: February 2004

We show that if ocean surface waves of the order of a few meters long are frequently steep enough to generate bound centimetric waves, then composite surface scattering theory can account for many anomalous properties of microwave backscatter from the sea at high incidence angles. The model proposed here postulates that these intermediate waves are made sufficiently steep to generate bound centimetric waves because of their modulation by longer, dominant ocean surface waves. The bound centimetric waves have a nonzero mean tilt because they are located on the steep forward face of the intermediate waves, and they move at the speed of the intermediate waves. Applying composite surface scattering theory to this sea surface model, we show that much of the apparently anomalous behavior of microwave sea return measured at incidence angles between 50° and 80° during the Synthetic Aperture Radar and X Band Ocean Nonlinarities‐Forschungsplatform Nordsee (SAXON‐FPN) experiment can be explained using reasonable parameters to characterize the surface waves. In the SAXON‐FPN measurements the mean values of the first moments of microwave Doppler spectra for horizontally polarized backscatter differ from those for vertically polarized backscatter by an amount which varies with the incidence angle and with the azimuthal angle between the radar look direction and the direction of the dominant wave. The modulation of this first moment by surface waves tens of meters in length is the same for the two polarizations at low to moderate incidence angles and can be interpreted in terms of the advection of free centimetric waves by the long waves. At higher incidence angles, however, this modulation is different for the two polarizations and cannot be explained by simple advection of free waves. Finally, microwave cross sections measured at high incidence angles using horizontal polarization are much larger than can be explained by a composite surface theory that includes only freely propagating centimetric waves. Most of these effects can be explained by the composite surface model presented here, which includes Bragg scattering from both free and bound, tilted waves.
Flow properties of the ice from the Greenland Ice Core Project ice core: The reason for folds?
Authors: Dahl‐Jensen D. , Thorsteinsson T. , Alley R. , Shoji H.
Published: February 2004

Long‐term deformation tests on ice from the Greenland Ice Core Project (GRIP) deep ice core show that ice from the different climate zones in the ice core has flow properties correlated with the concentrations of impurities in the sample. The deformation tests are performed by uniaxial unconfined compression at −16°C with an octahedral compression stress of 3 bars. The ice samples are compressed for ½ to 3 years until the tertiary strain rate is reached. It is believed that by the end all downhole flow conditions are forgotten and that the ice sample has settled in a state determined by the applied stress and temperature conditions. All samples are tested under the same stress and temperature conditions so the resulting deformation rates and final ice crystal size and fabrics can only differ due to varying impurity concentrations. The results show that ice from cold climatic periods with high concentrations of impurities deforms more slowly than ice from warm climatic periods in compression. When tertiary creep is reached, the crystal size is smaller in the cold ice than in the warm. The ice from warmer climatic periods with lower concentrations of impurities deforms at a factor of 2–3 times more rapidly in compression. The tertiary steady state crystal size is increased by 50% and the ice crystals have oriented more favorably for the applied compression in the warm ice, which is believed to be the reason why the strain rates are greater here than in the cold ice. In the bottom 200 m of the GRIP ice core, zones are observed with folds on the scale of 1–8 cm. An investigation of the ice layers in and around the folds shows that the layers are composed of ice from different climatic zones. The folding is believed to result from the different flow and rheological properties of the layers involved in the folding structures.
Ocean currents across the entrance to the Gulf of California
Authors: Collins Curtis A. , Garfield Newell , Mascarenhas Affonso S. , Spearman Monty G. , Rago Thomas A.
Published: February 2004

Observations of transport and currents in April, May, and December 1992 and January 1993 were made across the entrance to the Gulf of California with an acoustically tracked dropsonde. Flow was into the gulf along Sinaloa and out of the gulf along Baja California. The transports were 5–10 Sv and the currents were deep, with 10 cm s−1 flow extending to depths greater than 1000 m. The currents were intensified in the upper 300 m. Geostrophic flows compared well with observed currents when smoothed to 2–3 times the Rossby radius. Salinity in the upper 300 m was higher on the Baja California side of the gulf, indicating modification of Subtropical Subsurface waters within the Gulf as well as the presence of surface and near‐surface gulf waters. The salinity minimum associated with California Current waters at 50 m had narrow cores that can be resolved only with closely spaced conductivity‐temperature‐depth casts. Mass and heat fluxes for the upper 300 m were estimated as 280 t s−1 and −0.1×1012 W in May and 170 t s−1 and 2.0×1012 W in December.
A 110,000‐year history of change in continental biogenic emissions and related atmospheric circulation inferred from the Greenland Ice Sheet Project Ice Core
Authors: Meeker L. D. , Mayewski P. A. , Twickler M. S. , Whitlow S. I. , Meese D.
Published: February 2004

The 110,000‐year record of ammonium concentrations from the Greenland Ice Sheet Project 2 (GISP2) ice core provides the basis for an analysis of terrestrial biological production and atmospheric circulation patterns involved in the transport of biologically produced ammonium to the Greenland atmosphere. The directly measured concentration series was selected for analysis, rather than that of estimated ammonium flux, after a detailed analysis of the relationship among ice core glaciochemical concentrations and a high‐resolution simultaneous record of snow accumulation from the GISP2 core. Analysis of the ammonium concentration series shows that maxima in background levels of ammonium in the Greenland atmosphere are strongly related to and synchronous with summer forcing associated with the precessional cycle of insolation. Minima in background levels, on the other hand, are delayed relative to minima in summer insolation at those times when ice volume is significant. The duration of these delays are similar in magnitude (≈6000 years) to other paleoclimatic responses to changes in ice volume. Decadal and centennial scale variation about background levels of ammonium concentration exhibit two modes of behavior when compared to a record of polar atmospheric circulation intensity. During warmer periods ammonium transport to Greenland is similar to present patterns. Under coldest conditions the low levels of ammonium transported to Greenland are the result of extreme southerly excursions of the predominantly zonal polar circulation. The rapid transitions (≈200 years) between these two climatic conditions appear to be associated with a critical volume or extent of the continental ice sheets.
Analysis of the seasonal and interannual variability of the sea surface temperature field in the Adriatic Sea from AVHRR data (1984–1992)
Authors: Gačić M. , Marullo S. , Santoleri R. , Bergamasco A.
Published: February 2004

Seasonal and interannual variability of the sea surface temperature field in the Adriatic Sea is analyzed from the low‐resolution advanced very high resolution radiometer data. The spatial resolution of 18 km allowed analysis of only basin and subbasin scale features. Average monthly and seasonal sea surface temperature fields for the entire studied period (1984–1992) are discussed. The analysis shows the absence of any permanent sea surface thermal features in the Adriatic Sea. The south Adriatic sea surface temperature minimum presumably associated to the cyclonic gyre, previously considered as one of the permanent features, appears to be recurrent, being prominent only in late autumn and early winter, i.e., in the preconditioning and a deepwater formation phases. The major Ionian water inflow is documented in autumn while the thermal signature of the western surface outflow of Adriatic water appears most prominent in winter. The variability of the basin‐wide thermal pattern in the Adriatic reveals four distinct seasons, which is different from both the eastern and western Mediterranean, where only two major patterns are recognized. A prominent interannual signal occurs in a northward extension of the warm water plume along the eastern coast, which in some years reaches the northernmost corner of the Adriatic, while in other situations it remains trapped in the south Adriatic cyclonic gyre. The surface thermal signature of the south Adriatic gyre also varies on an interannual timescale, and it was weak or completely absent during the period 1984–1986 while it was rather prominent in the period 1987–1992. A constant trend of sea surface temperature decrease in the center of the south Adriatic gyre and in the northernmost corner of the Adriatic was evidenced over the studied period.
A numerical model of channel inception on submarine fans
Authors: Imran Jasim , Parker Gary , Katopodes Nikolaos
Published: February 2004

The most common type of submarine fan is created by the passage of a succession of depositional turbidity currents. It is thus of interest to note that even in this essentially depositional environment the fan surface is often intensely channelized. Here a two‐dimensional numerical model describing the formation of a submarine fan by a spreading turbidity current is presented. Layer‐averaged governing equations describing the turbidity current are presented and solved numerically in conjunction with the Exner equation of bed sediment continuity. The formulation describes fan evolution from an initially flat surface. The bed is allowed to evolve in response to the exchange of sediment with the turbidity current by means of simultaneous erosion and deposition of suspended sediment. The upstream boundary condition is chosen so as to approximate a submarine canyon debouching upon the fan. Under some but not all conditions the early stage of the depositional process is accompanied by the formation of incipient natural levees that act to partially channelize the flow, indicating the formation of a channel‐levee system. Dimensionless parameters are used to describe conditions for optimal channelization. The model can explain some of the basic mechanisms that account for the persistent tendency for channelization of the fan surface under a wide range of conditions.
A hydrodynamic ocean tide model improved by assimilating a satellite altimeter‐derived data set
Authors: Le Provost C. , Lyard F. , Molines J. M. , Genco M. L. , Rabilloud F.
Published: February 2004

An upgraded version of the tidal solutions (FES94.1) is presented, obtained by assimilating an altimeter‐derived data set in the finite element hydrodynamic model, following the representer approach. The assimilated data are drawn from the CSR2.0 Texas solutions sampled on a 5° × 5° grid. The assimilation is applied over the Atlantic, Indian, and Pacific Oceans. The standard release of the new FES95.2 solutions is a 0.5° × 0.5° gridded version of the full finite element solutions. The associated tidal prediction model includes 26 constituents. The eight major constituents are drawn directly from the hydrodynamic model: K1, O1, Q1, M2, S2, N2, K2, and 2N2, corrected by assimilation except K2 and 2N2. The other 18 constituents are derived by admittance. Among them are μ2, ν2, L2, T2, M1, P1, J1, and OO1. The quality of these solutions is evaluated by reference to a standard sea truth data set of 95 stations. This quality is significantly improved after the assimilation process is applied: the root‐sum‐square (RSS) of the differences between solutions and observations, for the eight major constituents, is reduced from 3.8 cm for FES94.1 to 2.8 cm for FES95.2, i.e., a gain of 1 cm. The performances of the prediction model are evaluated by comparing tidal predictions with observations at 59 sites distributed over the world ocean and by looking at the level of variance of the sea surface variability observed by the T/P altimeter at its cross‐over track points after tidal correction. These evaluations lead to the same conclusion: this new prediction model performs much better than the one based on FES94.1, because of correction of the major constituents by T/P data assimilation and because of the increase in the number of constituents from 13 to 26. The tidal predictions are at the level of accuracy of those produced by the best recent T/P empirical models.
Boundary versus internal diapycnal mixing in stratified natural waters
Authors: Goudsmit G.‐H. , Peeters F. , Gloor M. , Wüest A.
Published: February 2004

Using the fluorescent dye uranin, tracer release experiments to study the contribution of bottom boundary mixing to diapycnal transport in stratified natural waters were performed in Lake Alpnach (central Switzerland) during 1992–1995. A first experiment involved injecting the tracer from a point source into the center of the hypolimnion (that part of the lake below the surface mixed layer). An in situ fluorometer was then employed to detect the horizontal and vertical spreading of the tracer cloud, allowing rates of diapycnal diffusivity to be determined. As long as the tracer was confined to the interior water region, the diapycnal diffusivity was relatively small. However, after the tracer cloud had reached the lake boundary, the diapycnal diffusivity increased by approximately one order of magnitude. In a second experiment, the tracer was released near the sediment‐water interface. In this case the dynamics of vertical tracer spreading were opposite. During the first few hours after tracer release, diapycnal diffusivities were large, subsequently decreasing as the tracer cloud drifted away from the lake boundary. Basin‐wide diapycnal diffusivities calculated from heat flux measurements based on temperature profiles obtained from thermistor chains or conductivity‐temperature‐depth casts agreed well with the values obtained from the vertical tracer diffusion after horizontal homogenization. The results of the tracer experiments corroborate the hypothesis that diapycnal fluxes are determined predominantly by mixing in the bottom boundary region.
Variations in mean currents affecting hydrothermal plumes on the Juan de Fuca Ridge
Authors: Cannon Glenn A. , Pashinski David J.
Published: February 2004

A summary of current meter observations near the Juan de Fuca Ridge shows new features of topographic steering of flow that determine the fate of plumes from hydrothermal vents. An anticyclonic circulation exists over the ridge with north and south mean flows along the west and east flanks. Speeds are up to 2–3 cm s−1 near the ridge and decrease to zero about 25 km from the crest. Contributions from hydrothermal forcing and tidal rectification may generate the along‐ridge flows. Average anomalous heat fluxes associated with the north and south flows were estimated at 1400 and 2600 MW, respectively, and they are larger than previous estimates based on single‐point current measurements. Variability of flow occurs on several time scales and space scales. Daily average currents over a 4‐day period can rotate over the entire ridge with sufficient cross‐axis speeds to move plumes away from the vents into the stronger north‐south flows over the flanks. Currents along the west side of North Cleft diverge with near‐ridge flow (<10 km from the ridge) turning eastward around the north end of the segment and offshore flow turning westward. Farther west in winter, off‐axis currents can be several times as large as the long‐term mean and could advect plume water relatively long distances. Significant cross‐axis flow occurs on the north side of Axial Volcano that may be the result of interaction between the general circulation and topography generating eddy circulations around or near Axial or the entire Axial‐Cobb massif.
Altimeter sampling characteristics using a single satellite
Authors: Parke Michael E. , Born George , Leben Robert , McLaughlin Craig , Tierney Craig
Published: February 2004

Altimetric satellites have characteristic sampling patterns in both space and time based on their repeat period and orbit inclination. Aliased phenomena measured by altimetric measurements can appear as propagating waves with both wavelength and direction of propagation different from the underlying phenomena. All signals that contribute to the altimetric measurement can be aliased and produce such patterns, not just tidal signals. For example, mesoscale energy will be aliased as will unmodeled atmospheric variations. Past discussions of aliasing have only considered spatially homogeneous signals. This paper extends this work to phenomena with finite wavelengths and considers both the north‐south and east‐west components of the resulting aliases.
Coastal lee waves on ERS‐1 SAR images
Authors: Zheng Quanan , Yan Xiao‐Hai , Klemas Vic , Ho Chung‐Ru , Kuo Nan‐Jung , Wang Zongming
Published: February 2004

Alternative dark‐bright patterns on ERS‐1 synthetic aperture radar (SAR) images of the west side of the Taiwan Strait taken on December 8, 1994, were recognized to be the sea surface signature of a coastal lee wave. Such waves are called coastal lee waves because they occur along the lee side of the coast. The coastal lee waves appeared in the form of a wave packet distributed within an offshore band 20–40 km wide. The first packet, which occurred in the northern portion of the observed area, contained six waves with variable wavelengths (defined as the spatial separation between two waves) from 1.7 to 2.7 km. The second packet, in the middle, contained 10 waves with a relatively uniform wavelength of 4.2 km. The third packet, in the southern portion, contained 17 waves with an average wavelength of 2.0 km. The crest lengths were from 20 to 80 km. Local meteorologic parameters observed simultaneously at Fuzhou, China, close to the imaged area, showed an offshore wind of 1.5–3.5 m/s and a land surface air temperature of 19°C, which was 4°C lower than the sea surface temperature (SST). Thus the lower atmospheric boundary conditions at imaging time were very favorable both for generating the land breeze circulation and small wind waves on the sea surface, which are in the Bragg‐scattering wavelength band of the C band ERS‐1 SAR. A physical model of a three‐layer atmosphere was developed in order to explain how the land breeze circulation can generate the coastal lee waves. The results showed that the vertical velocity disturbance caused by the wind convergence at the land breeze frontal zone is of vital importance for the generation of coastal lee waves, and the model gave very good estimates of the processes observed. The SAR imaging mechanisms of the waves were analyzed in detail. The differences between coastal lee waves and ocean internal waves, which appear as similar alternative dark‐bright patterns on SAR images, were also discussed.
Distribution of helium and tritium in Lake Baikal
Authors: Hohmann R. , Hofer M. , Kipfer R. , Peeters F. , Imboden D. M. , Baur H. , Shimaraev M. N.
Published: February 2004

The 3H‐3He age of a water mass is a measure of the time that has passed since the water mass was last in contact with the atmosphere. Between 1992 and 1995 a detailed study of 3H‐3He ages was conducted in Lake Baikal, the deepest and largest lake by volume on Earth, to investigate deep water renewal in its three major basins. Maximum 3H‐3He ages are 14–17 years in the southern basin, 16–18 years in the central basin, and 10–11 years in the northern basin. Rates of renewal of deep water with surface water, deduced from volume‐weighted mean 3H‐3He ages below 250 m depth, are about 10% yr−1 in the southern and central basins and 15% yr−1 in the northern basin. In the southern basin the mean 3H‐3He age below 250 m depth increased steadily from 9.6 years in 1992 to 11 years in 1995, indicating a slight diminution in deep water renewal during this time. Bottom water renewal by large‐scale advection was estimated from the mass balance of 3He in the 200 m thick bottom layer of each basin. Bottom water renewal rates in the northern basin were found to be between 80 and 150 km3 yr−1 and in the central basin between 10 and 20 km3 yr−1, whereas in the southern basin they were practically zero. Correlating oxygen and dissolved helium‐4 concentrations with 3H‐3He age allowed us to determine the mean hypolimnetic oxygen depletion rate in the water column (4.5 μmol L−1 yr−1), as well as mean helium fluxes from the lake bottom (2.8×1011 atoms m−2 s−1 in the northern basin, and 1.3×1011 atoms m−2 s−1 in the central and southern basins). The helium isotope ratio of the terrigenic helium component injected from the lake bottom, determined from measurements of water from hydrothermal springs in the vicinity of the lake, was found to be ∼2.2×10−7.
Lead convection measured with an autonomous underwater vehicle
Authors: Morison James H. , McPhee Miles G.
Published: February 2004

The winter Lead Experiment (LeadEx) was conducted during 1992 in the Beaufort Sea. There the Autonomous Conductivity Temperature Vehicle (ACTV) was operated under and around leads. These are the first measurements of this kind. Measurements of temperature, salinity, and turbulence parameters were also made at lead edges with sensors fixed to rigid masts. By combining information from the fixed measurements and the ACTV an inertial dissipation method is used to derive horizontal profiles of salt flux from ACTV salinity measurements. Comparisons also show that turbulent vertical velocity perturbations can be estimated from the vertical motion of the ACTV. With these velocity estimates, horizontal profiles of the turbulent fluxes of heat, w′T′, and salt, w′S′, are also computed directly. Data from a wide (1000‐m‐wide) lead with rapid ice motion indicate stronger turbulence under the lead than downstream, but the character of the flow is that of a convective boundary layer modified by a change in surface buoyancy flux. The convection is characterized by multiple, intermittent plumes that scale in size with the mixed layer depth. At a medium‐sized (100‐m‐wide) lead with slower ice motion the convective circulation set up by the lead is more apparent. Convective plumes are stronger and more prevalent at the lead edges, particularly the downstream edge. The comparison between the horizontal profiles from the ACTV and time series from fixed sensors suggests that lead convection is a combination of a near surface convective boundary layer and a deeper quasi‐stationary convection pattern.
Seasonal transport variability of the Deep Western Boundary Current in the equatorial Atlantic
Authors: Fischer Jürgen , Schott Friedrich A.
Published: February 2004

A total of 21 about year‐long current meter records in the depth range of the upper and middle North Atlantic Deep Water (NADW) were analyzed to determine the mean and the fluctuations of the upper Deep Western Boundary Current (DWBC) in the equatorial Atlantic. The investigation was based on moored arrays at 44°W from three different deployment periods, 1989/1990, 1990/1991 and 1992/1994, and was supplemented by current profiling along 44°W and 35°W. The approximately 100‐km‐wide DWBC at 44°W, just north of the equator, was attached to the topography with the current maximum exceeding 70 cm s−1. Currents within the DWBC core followed the topography, and the close agreement between the mean current direction and the direction of maximum variance indicated that the major contribution to the DWBC variability near the equator was due to pulsing rather than meandering. For mean transports of upper and middle NADW, the current meter records were averaged over their deployment duration yielding a best estimate of 13 Sv in the depth range 1000 to 3100 m. The mean transport appeared robust, as subsets of the data from two different years yielded about the same mean transport, namely, 12.4 and 13.6 Sv. The DWBC transport time series showed a definite seasonal cycle, ranging from less than 7 Sv during September/October to about 25 Sv during January/February. Annual and semiannual transport harmonics had similar amplitudes, at about 6 Sv each, and together they explained about two thirds of the total transport variability. After crossing the equator, the DWBC splits into two cores with the major flow along a chain of seamounts near 3.5°S, near 35°W. Magnitudes and phases of the transport variability at 35°W, south of approximately 1.5°S, were similar to that at 44°W. Further, for the flow of lower NADW which was detached from the upper DWBC core, similar periodicity and phases were observed in the deep records at 44°W.
Sensitivity of a wave model to wind variability
Authors: Ponce Sonia , Ocampo‐Torres Francisco J.
Published: February 2004

A third‐generation wave model was applied to the Gulf of California to hindcast wave spectra and to assess model sensitivity to wind variability on the basis of three different numerical simulations: (1) when a synthetic wind field of varying randomness is adopted and when the forcing wind field considered has an input time step of (2) 6 hours and (3) 5 min. In the first idealized simulation the wave field induced by a constant wind field is compared with the result when white noise was added to the originally constant forcing winds. Results from these numerical simulations demonstrate that wave energy increases with wind variability, even though the mean wind is kept constant. In the second and third simulations the forcing wind is averaged for periods of 6 hours and 5 min, which represent relatively low and high timescale variability, respectively. These realistic wind fields were constructed as a blend of detailed in situ measurements and analysis information, representing wind variability in both speed and direction. The results show that high‐variability winds induce broad directional wave spectra and secondary peaks with similar magnitude as the main peak (bimodal spectra). With the presence of wind gustiness and a continuous spectrum of wind variability, all source terms are expected to play a significant role in the evolution of the wave spectrum.
Capture and release of Lagrangian floats by eddies in shear flow
Authors: Shapiro G. I. , Barton E. D. , Meschanov S. L.
Published: February 2004

A mechanism to explain the observed trapping or release of Lagrangian drifters by eddies is proposed, based on an interaction between the eddy circulation and the background flow. Eddies strong in relation to the background have a large “trap zone” within which particles are constrained to circulate in closed paths about the center, while relatively weak eddies have a smaller trap zone. Particles outside the trap zone drift freely, while those inside rotate with the eddy. As an eddy is advected by non‐uniform shear flow, its ‘trap zone’ expands or contracts so particles previously free become trapped or trapped particles are freed. Numerical experiments with idealized eddies embedded in different flows illustrate the concept. It is shown that drifters may begin to loop, as in some previously reported observations, without any need for eddy generation. Similarly, they may cease looping, without eddy destruction. The particular case of a strong cyclonic eddy south of Gran Canaria is modeled, using the observed density field and two drifter trajectories. Predicted drifter trajectories are compared with the observations. It is shown that the trajectories of buoys launched near the edge of the eddy are sensitive to small variations in the background field.
Ocean modeling for ENSO
Authors: Stockdale Timothy N. , Busalacchi Antonio J. , Harrison D. E. , Seager Richard
Published: February 2004

Tropical ocean modeling has played a major part in the development of our knowledge of El Niño‐Southern Oscillation (ENSO) during the Tropical Ocean‐Global Atmosphere (TOGA) decade. Although the foundations had already been laid, it was only with the impetus from TOGA that tropical ocean modeling was able to develop so extensively. In this paper we discuss the development of the wide range of tropical ocean models in use today, from the simple to the complex; the ways in which their abilities to reproduce different phenomena have been assessed; and the ways in which they are being used to better understand and predict the behavior of the coupled ocean‐atmosphere system in the tropics. Ocean model development is far from finished, however. Outstanding issues such as forcing fields, model improvements, testing strategies, and applications are also discussed. There is at least as much work to be done in the next decade as was achieved in the last.