bluuebunny/crossref_metadata_2025 huggingface dataset:
Fisheries and oceanography off Galicia, NW Spain: Mesoscale spatial and temporal changes in physical processes and resultant patterns of biological productivity
Authors: Tenore K. R. , Alonso‐Noval M. , Alvarez‐Ossorio M. , Atkinson L. P. , Cabanas J. M. , Cal R. M. , Campos H. J. , Castillejo F. , Chesney E. J. , Gonzalez N. , Hanson R. B. , McClain C. R. , Miranda A. , Roman M. R. , Sanchez J. , Santiago G. , Valdes L. , Varela M. , Yoder J.
Published: February 2004

The Galician shelf off NW Spain (43N° 9W°) exhibits mesoscale spatial and temporal changes in biological productivity associated with upwelling. Spatial heterogeneity results from local geomorphic and land‐sea interactions superimposed on the large scale atmospheric processes that produce upwelling. Wind‐induced upwelling events, commonly of short (i.e., week) duration, are more common in the summer than in the winter. A series of cruises, including some time series sampling, and satellite imagery analysis showed that surface upwelling was more common and persistent on the northern coast compared with the western coast off the coastal embayments, the Rias Bajas. Near shore off the rias, coastal runoff, which is greater in the rainy winter/spring versus the dry summer, affected upwelling. In early summer, upwelling less often reaches the surface because of increased water column stratification associated with lower surface salinities and thus upwelling is not detected by satellite imagery. Conversely, in late summer, upwelling more often reaches the surface because coastal runoff is reduced during the dry summer months and the water column tends to be less stratified. Plankton biomass and rate processes along the Galician shelf reflected both ambient hydrographic conditions as well as prior history of upwelling or downwelling. Phytoplankton and bacterioplankton were in greatest abundance during upwelling conditions (June through August); in contrast, both Zooplankton and fish larvae exhibited highest abundances in March, when there were upwelling conditions prior to our cruise. Spatial differences in the duration and frequency of upwelling events, in combination with advection of water masses, are critical to the patterns of water column productivity and sardine fisheries production off the Galician coast. More persistent upwelling at this NW corner of the Iberian peninsula supports large sardine fisheries because Zooplankton and larval fish populations have time to respond to the higher primary production. Farther down the western Galician coast, the episodic upwelling and resultant intermittent primary production does not support a stable food supply needed to support fisheries. Times series sampling revealed mean response times of bacteria, phytoplankton, and Zooplankton to be on the order of a day, days, and weeks, respectively. Sardines showed no spawning response in the relatively short time series sampling. The observed distributional patterns of fish eggs and larvae showed some offshore transport of fish larvae that were spawned inshore during upwelling periods and aggregation of larvae in a convergence zone northwest of Cabo Villano.
The influence of the seasonal mixed layer on oceanic uptake of CFCs
Authors: Haine Thomas W. N. , Richards Kelvin J.
Published: February 2004

The issue of the appropriate GFC boundary condition for the interior of the ocean is addressed. A review of observations clearly shows substantial departure from saturation equilibrium and represents the greatest uncertainty in interpreting CFG measurements. A series of simple models is developed to determine the factors which control the CFC‐11, CFC‐12 and CFC‐113 saturations and CFC‐113:CFC‐12 ratio age.The physical processes of subduction and entrainment have a strong effect; the sequestration of fluid in the seasonal thermocline and spatial variations in the depth of mixing are most important. The uncertainty in the chemical mechanisms has a smaller influence. To the limited extent that the observational database can be compared to the model results, there is good agreement. The model suggests that the variation in CFC‐113:CFC‐12 age is significantly less than in the saturations of CFCs 11, 12, or 113. However, given a physically realistic seasonal ocean model, the CFC saturation and age boundary condition should be predictable.
On the interpentadal variability of the North Atlantic Ocean: Model simulated changes in transport, meridional heat flux and coastal sea level between 1955–1959 and 1970–1974
Authors: Ezer Tal , Mellor George L. , Greatbatch Richard J.
Published: February 2004

Previous studies by Greatbatch et al. (1991) indicate significant changes in the North Atlantic thermohaline structure and circulation between the pentads 1955–1959 and 1970–1974, using data analyzed by Levitus (1989a,b,c) and a simple diagnostic model by Mellor et al. (1982). In this paper these changes are modeled using a three‐dimensional, free surface, coastal ocean model. Diagnostic and short‐term prognostic calculations are used to infer the dynamically adjusted fields corresponding to the observed hydrographic and wind stress climatology of each pentad. While the results agree with earlier studies indicating that the Gulf Stream was considerably weaker (by about 30 Sv) during the 1970s compared to the 1950s, they also indicate some changes in the poleward heat transport, although the statistical significance of these changes relative to sampling errors in the data is not clear. The change of wind pattern between the two pentads, associated with changes in sea surface temperature, resulted in changes in the Ekman contribution to the poleward heat flux transport. The modeled sea level along the North American coast shows a sea level rise of about 5–10 cm between 1955–1959 and 1970–1974; a comparison with observed sea level at 15 tide gage stations shows good agreement. Most of the coastal sea level change is attributed to changes in thermohaline ocean circulation and wind stress; thermal expansion seems to play a lesser role. The methodology tested here demonstrates an effective way to estimate climate changes in ocean circulation and sea level from observed hydrographic data and winds using ocean models to enhance and analyze the data.
Buoy observations of the atmosphere along the west coast of the United States, 1981–1990
Authors: Dorman Clive E. , Winant Clinton D.
Published: February 2004

The distribution of statistical properties of the meteorological and sea surface temperature fields along the west coast of the United States is described based on 10‐yearlong observations from buoys deployed by the National Data Buoy Center. The observations suggest that properties vary differently in each of three different regions along the coast: the Southern California Bight which remains sheltered from strong wind forcing throughout the year; the central and northern California coast up to Cape Mendocino, the site of persistent equatorward winds; and the Oregon‐Washington coast, where traveling cyclones and anticyclones produce vigorous and variable forcing. Over most of the region the variance in the wind speed is roughly equally divided between the annual cycle and the synoptic forcing, corresponding to periods between 5 and 50 cycles per year. Two seasons, summer and winter, are sufficient to describe the annual cycle. During the summer, two distinct wind speed maxima occur along the coast, one near Point Conception and the other off northern California, between Point Reyes and Point Arena. In the winter a single maximum occurs, located near Point Conception. The atmospheric pressure generally increases with latitude along the coast, but the annual cycle of atmospheric pressure has a different phase, depending on location; off the coast of California, highest pressures are found during the winter, while off Oregon and Washington, the highest pressures occur during the summer. Fluctuations in air and sea temperature are highly correlated, and the sea temperature is usually higher than the air temperature, in the winter. Examination of vertical soundings of the atmosphere at Oakland, Vandenberg Air Force Base and San Diego during the same period of time reveals that a well‐defined inversion separates the marine boundary layer (MBL) from the free atmosphere above nearly 90% of the time during the summer and half the time during the winter. Station soundings consistently overestimate the MBL thickness, but the results do suggest that the MBL is supercritical part of the time in the vicinity of the three sites. An attempt is made to examine the interannual variability and compare it to the Southern Oscillation index, although the results are limited because the record length is short compared with interannual timescales. Spatially averaged temperature anomalies increase during winter 1982–1983, coincident with the large El Niño event.
Tidal and lower frequency currents above Fieberling Guyot
Authors: Brink K. H.
Published: February 2004

An intensive, yearlong deployment of current meter moorings was made over and around Fieberling Guyot (32.5°N, 127.75°W) in the North Pacific. The measurements are used to investigate tidal currents, the mean flow, and subinertial current fluctuations. The K1 and O1 diurnal tides were greatly amplified over the seamount, apparently through dynamics resembling nonlinear seamount‐trapped waves. The diurnal tides interacted nonlinearly to induce constituents at the sum frequency (M2) and difference frequency (fortnightly) above the summit. Diurnal amplification was strongest at about 450 m, and it extended from the seamount center out over the flanks. The S2 semidiurnal tide was not substantially amplified over the seamount. The mean flow was primarily along isobaths and was strongest about 50 m above the seamount rim. It was driven by tidal rectification, so that mean along‐isobath velocity at a given location correlated well with the amplitude of the local fortnightly tide at that location. Accompanying the mean flow was a “cold dome” over the seamount that was presumably maintained by a balance of mean downward advection and eddy (tidal) radial heat transport. Lower frequency current fluctuations were enhanced near the seamount and were apparently driven by fluctuations in the ambient flow.
The anatomy of the Arctic Frontal Zone in the Greenland Sea
Authors: van Aken Hendrik M. , Budéus Gereon , Hähnel Michael
Published: February 2004

From a series of two‐dimensional expendable bathythermograph (XBT) surveys, conductivity‐temperature‐depth (CTD) sections, surface drifters, and acoustic Doppler current profiler (ADCP) observations, the properties and structure of the Arctic Frontal Zone in the Greenland Sea have been determined. The Arctic Frontal Zone appears to be a large‐scale, climatic “multifrontal” frontal zone. The structure of the frontal zone can be discerned from subsurface as well as from surface hydrographic parameters, even in summer, when a seasonal thermocline covers the subsurface hydrographie structure. The Arctic Frontal Zone consists of two semipermanent frontal interfaces with warm, saline Norwegian Atlantic Water to the east and Arctic Water from the Greenland Sea gyre to the west. The two frontal interfaces are bounding a band of shallow cyclonic cold eddies and anticyclonic warm eddies with horizontal scales of the order of 40–50 km. The typical diameter of the eddies can be scaled with the local internal Rossby radius of deformation. The eddy kinetic energy of the surface flow in the frontal zone is of the order of 60 to 85 cm2 s−2. The zonal density gradient in the Arctic Frontal Zone maintains a mean northward geostrophic transport of 3.8 Sv, averaged over a number of cruises. This transport is mainly connected with the frontal interface on the western side of the warm and saline Norwegian Atlantic Water. The estimated cross‐frontal eddy transports of heat and salt appear to be of considerable importance for the conditioning of the Greenland Sea gyre.
Recirculation in the lee of complicated headlands: A case study of Bass Point
Authors: Denniss Tom , Middleton Jason H. , Manasseh Richard
Published: February 2004

It has been argued in recent papers that the nature of headland and coastal island wakes can be highly dependent on the level of turbulence caused by complicated coastal geometry and bottom topography, as well as on the loss of energy and momentum to the seabed due to friction. A series of field experiments was conducted around Bass Point (a 4‐km‐long headland near Sydney, Australia) using moored current meters and an acoustic Doppler current profiler. The offshore component of currents, measured offshore of the point, showed a high degree of spectral energy at about the 7‐day cycle, with a small amount at the diurnal frequency, but the reverse was found in the lee of the headland, where most of the recirculation was at the diurnal frequency. Numerical experiments resulted in similar spectra being obtained from locations behind the headland when the numerical model was forced with a spectrum of frequencies resembling the spectrum of the offshore free stream current, which was derived from the field results. The size and frequency of eddyshedding in the lee of the headland depended on the degree of variability of the inflowing free stream, with a diurnal component in the free stream appearing to cause the oscillations in the wake also of diurnal frequency. The numerical experiments also show that the size of the recirculating wake was affected by the complexity of the headland's geometry.
Internal wave directional spectra using an acoustic Doppler current profiler
Authors: Marsden R. F. , Juszko B.‐A. , Ingram R. G.
Published: February 2004

An acoustic Doppler current profiler was deployed through land‐fast ice near Resolute, Northwest Territories. It recorded a distinct train of high‐frequency (periods < 20.0 min) internal waves lasting 10 hours. Assuming linear dynamics, a modal decomposition of the three velocity channels led to the modal pressure anomaly taking a role analogous to that of the surface displacement sensed by a surface following heave, pitch, and roll buoy. Partitioning the data into frequency dependent cross‐spectral matrices of the modal horizontal pressure anomaly enabled the computation of high‐resolution, data‐adaptive directional spectra, recently presented for use with surface gravity waves. The mode 1 directional spectra were particularly sharp and indicated a cross‐channel point of origin of the waves to the northeast of the study site. The lowest two frequency bands contained considerable spectral power density that could not be accurately described by the modal decomposition. A direct examination of the cross‐channel velocity time series indicated substantial low‐frequency (periods > 30.0 min) horizontal flow without a corresponding coherent vertical velocity component, attributed to a density driven along‐isopycnal flow created by salt rejection emanating from a broad shelf to the northeast.
The mean structure and variability of the Mindanao Current at 8°N
Authors: Wijffels Susan , Firing Eric , Toole John
Published: February 2004

On the basis of eight surveys along 8°N in the western Pacific Ocean taken between September 1987 and June 1990, the mean structure and transport of the Mindanao Current is presented. Direct acoustic Doppler current profiler (ADCP) velocity measurements show the Mindanao Current to be an intense 200‐km‐wide coastally trapped jet with a typical inshore velocity maximum of 1 m s−1. Though the individual geostrophic velocity estimates were strongly contaminated by dynamic height noise, the survey averaged geostrophic and directly measured shears were identical to within the estimated errors. The ADCP mean velocity field was taken as a reference for the geostrophic flow estimate based on the; mean density distribution. Relative to a bottom reference level, a barotropic correction was required near the coast to make the mean geostrophic and directly measured velocities agree. Integrated from the Mindanao Coast to 130°E, the average transport above σθ = 26.7 kg m−3 is 25±4×109 kg s−1, while that between 0 and 2000 dbar is 39 ±10×106 kg s−1 based on the mean ADCP‐referenced geostrophic fields. The velocity structure of the Mindanao Current was remarkably stable from cruise to cruise. However, two cruises in the spring of 1989 sampled an anomalous velocity structure offshore which is attributed to the presence of a 400‐km‐diameter warm‐core feature containing South Pacific water masses. We speculate that this feature is either a pinched‐off eddy or meander from the confluence of northern and southern hemisphere waters that occurs near 5°N. The sense of circulation of the eddy is anticyclonic, opposite in sign to the recirculation which has been historically termed the Mindanao Eddy. Only two of the eight cruises sampled northward thermocline flow offshore of the Mindanao Current; the average of all cruises contains no significant recirculation. Hence for the time period of these surveys the Mindanao Eddy and Undercurrent were essentially absent at 8°N.
Detail and scaling of turbulent overturns in the Pacific Equatorial Undercurrent
Authors: Peters Hartmut , Gregg Michael C. , Sanford Thomas B.
Published: February 2004

We analyze turbulent overturns in the high‐shear, low Richardson number flow of the upper 350 m at 0°/140°W. Profiles of shear and stratification combined from fine‐ and microscale sensors resolve the vertical wavenumber spectrum from large‐scale to dissipation scales. We compute a turbulent length scale l from Thorpe‐sorting potential temperature, while using potential density to avoid thermohaline intrusions. Pragmatically, we consider scales smaller than l turbulent and scales large than l nonturbulent. From local fine‐scale velocity spectra, we extract the horizontal turbulent kinetic energy at scales smaller than l and thus estimate the turbulent velocity q, a parameter characteristic of the energetic eddies. The independently observed viscous dissipation rate ε, q, and l follow Taylor scaling, ε = cq q3/l, with cq ≈ 4. Similarly, the measured thermal dissipation rate χ and the turbulent temperature fluctuation T′, also estimated by spectral extraction at scales smaller than l, follow similarity scaling, χ = ctT′2q/l, with ct ≈ 7. From q, l, buoyancy frequency N, and kinematic viscosity ν, we estimate turbulent Reynolds numbers Ret = ql/ν and turbulent Froude numbers Frt = q /(Nl). The more energetic overturns of vertical thickness exceeding 1 m have 0.1 ≲ Frt ≲ 3 and 250 ≲ Ret ≲ 105. Ozmidov scales follow overturning scales as described by Dillon (1982) only on average, but not in individual overturns. Richardson numbers Ri of overturns show large scatter around a median of Ri = 0.23 and virtually no correlation with ε. The mixing efficiency shows a weak increase with increasing Ri and a weak decrease with increasing Frt. Turbulence parameters are briefly compared with formulations in turbulence closure models. A few individual mixing events are analyzed in detail, with focus on possible forcing mechanisms. In such identifiable events, enhanced turbulence is paralleled by enhanced fine‐scale variance of velocity, shear, and temperature. One mixing event shows signatures of critical layer absorption. A very large overturn in the nighttime turbulent layer near the surface surprisingly shows dissipation rates indistinguishable from the surroundings.
A numerical study of three‐dimensional dense bottom plumes on a Southern Ocean continental slope
Authors: Jiang Lin , Garwood Roland W.
Published: February 2004

Three‐dimensional features of dense bottom plumes flowing over continental slopes, with or without along‐slope topographic variations, are investigated by simulating the evolution of a density front at a Southern Ocean continental shelf break, using a three‐dimensional, primitive equation numerical model with a second‐order turbulence closure scheme embedded. The focus of our investigation is the role of topography in determining mixing and the offshore transport of dense shelf water during the transient adjustment process of a density front over a continental slope. We compare and discuss the numerical simulations for two cases: a uniform shelf and slope case and a case with a canyon that leads from the coast to the deep ocean crossing the shelf and the slope. The numerical simulations indicate that baroclinic instability, planetary rotation, bottom friction, and topography play major roles in determining the surface and bottom plume formation, the growth and penetration depth of the bottom plumes, and the characteristics of water mass on the slope. Mesoscale eddies play a fundamental role in transporting mass, heat, and salt from shelf to deep ocean. The effects of the depth and width of the canyon are examined with two more experiments. The presence of a wide and deep canyon in the continental shelf and slope enhances considerably the drainage of coastal shelf water into the deep ocean.
On the parameterization of equatorial turbulence: Effect of fine‐scale variations below the range of the diurnal cycle
Authors: Peters Hartmut , Gregg Michael C. , Sanford Thomas B.
Published: February 2004

We seek relationships among turbulence and fine‐scale and large‐scale flow by ensemble averaging the observations taken in the Pacific Equatorial Undercurrent (EUC) at 0°/140°W in April 1987. A combination of fine‐ and microscale sensors resolves vertical wavenumber spectra of shear and temperature from scales of 300 m to the viscous and thermal diffusive cutoffs. We study the depth range 50–350 m, which encompasses high‐shear layers below and above the core of the EUC and the thermostad, but not the diurnal cycle near the surface. Fine‐scale shear dominates over large‐scale shear (related to the slowly varying EUC) at 50–170 m and below 270 m, where large‐scale gradient Proude numbers (
) drop below 1, while large‐scale shear dominates in the weakly stratified thermostad at 170–270 m, where
> 1. We analyze shear fluctuations in different vertical wavenumber bands and pragmatically separate nonturbulent and turbulent fluctuations, the latter being associated with vertical overturning and viscous dissipation. In part of the fine‐scale range, shear spectra fall off approximately in inverse proportion to vertical wavenumber. The shear variance in this wavenumber band stays close to the squared buoyancy frequency independent of large‐scale shear. At a vertical resolution that resolves turbulent overturning, the Kunze et al. (1990) model of shear instability well predicts average dissipation rates below 100 m. Yet instantaneous high‐resolution gradient Froude numbers show virtually no correlation with turbulent dissipation rates. At 20‐m vertical resolution, mean dissipation rates from below 50 m and total rms shear Stot are well correlated as
. Fine‐scale shear is essential in this relationship. Large‐scale gradient Froude numbers and large‐scale shear are comparatively poorly correlated with mixing parameters.
Fine structure, microstructure, and vertical mixing processes in the upper ocean in the western Weddell Sea
Authors: Robertson Robin , Padman Laurie , Levine Murray D.
Published: February 2004

The upward flux of heat from the subsurface core of Warm Deep Water (WDW) to the perennially ice‐covered sea surface over the continental slope in the western Weddell Sea is estimated using data obtained during February–June 1992 from a drifting ice station. Through the permanent pycnocline the diapycnal heat flux is estimated to be about 3 W m−2, predominantly because of double‐diffusive convection. There is no evidence that shear‐driven mixing is important in the pycnocline. The estimated mean rate of heat transfer from the mixed layer to the ice is 1.7 W m−2, although peak heat fluxes of up to 15 W m−2 are found during storms. It is hypothesized that isopycnal mixing along sloping intrusions also contributes to the loss of heat from the WDW in this region; however, we are unable to quantify the fluxes associated with this process. Intrusions occur intermittently throughout this experiment but are most commonly found near the boundary of the warm‐core current and the shelf‐modified water to the east. These heat fluxes are significantly lower than the basin‐averaged value of 19 W m−2 (Fahrbach et al., 1994) that is required to balance the heat budget of the Weddell Gyre. Other studies suggest that shelf processes to the west of the ice station drift track and more energetic double‐diffusive convection in the midgyre to the east could account for the difference between our flux estimates for this region and those based on the basin‐scale heat budget.
Upwind‐weighted advection schemes for ocean tracer transport: An evaluation in a passive tracer context
Authors: Hecht Matthew W. , Holland William R. , Rasch Philip J.
Published: February 2004

Centered‐in‐space, centered‐in‐time integration has generally been used for the advection of scalars in ocean models. An assessment is made of the implications of centered leapfrog integration in the context of two‐dimensional passive tracer advection within a Stommel (1948) gyre. Nonphysical ripples in the tracer field grow to alarming levels in purely advective integrations. Diffusive parameterizations of eddy mixing moderate these ripples, but it is found that Laplacian diffusion greatly reduces the peak amplitude of the tracer field, while biharmonic or weaker Laplacian diffusion allows ripples of large amplitude. Several forward‐in‐time, upwind‐weighted schemes are found to provide better solutions. Smolarkiewicz's (1984) Multi‐Dimensional Positive‐Definite Advection and Transport Algorithm (MPDATA) scheme is slightly superior for an integration at moderate resolution within which the western boundary current is poorly resolved in typical fashion. Third‐order, upwind‐based schemes exhibit little sensitivity to the details of multidimensional treatment for this problem of passive tracer advection, with results nearly as good as for MPDATA.
The sensible heat of rainfall in the tropical ocean
Authors: Gosnell R. , Fairall C. W. , Webster P. J.
Published: February 2004

Precipitation affects the buoyancy of the upper ocean by altering the salinity and temperature distribution. If precipitation exceeds evaporation, salinity decreases and the buoyancy of the upper ocean increases. If the temperature of the rain drops and the sea surface temperature are different, precipitation induces a sensible heat flux into the ocean. Generally, it is assumed that the rain drop temperature is close to the sea level wet‐bulb temperature. However, because of the finite thermal relaxation time of large rain drops, the temperature of the rain drops could be colder than the sea‐level wet‐bulb temperature and the sensible heat flux enhanced. We test this possibility by developing a microphysical model to compute the temperature correction from first principles. For tropical warm pool conditions the temperature correction (i.e., difference between rain drop temperature and the wet‐bulb temperature) has a maximum of only 0.2 K for normal conditions. The theoretical calculation thus corroborates the general assumption. The model is used to calculate sensible heat flux contributions with rain rate and surface meteorological data to estimate the rain cooling obtained during the Tropical Ocean‐Global Atmosphere Coupled Ocean‐Atmosphere Response Experiment (TOGA‐COARE). The average heat flux from rainfall over the entire TOGA‐COARE period for the rain was approximately 2.5 W m−2. However, during intense rainfall events the rainfall sensible heat flux may be as high as 200 W m−2. During these periods the magnitude of the rainfall sensible heat flux rain cooling matches that of the latent heat flux. Because of the uncertainties in measuring meteorological parameters and because the heat flux calculated from the wet‐bulb temperature differed by only about 0.1 W m−2, the wet‐bulb temperature is still a reasonable value to use for the temperature of falling rain in sensible heat flux computations.
Gulf Stream path and thermocline structure near 74°W and 68°W
Authors: Watts D. Randolph , Tracey Karen L. , Bane John M. , Shay Thomas J.
Published: February 2004

The SYNoptic Ocean Prediction (SYNOP) experiment had the goal of providing a physical understanding of energetic mesoscale eddy processes in the Gulf Stream. In the SYNOP Inlet Array off Cape Hatteras and in the Central Array near 68°W modred observations were collected from October 1987 through August 1990. The Inlet Array measured the surface path and bottom currents where the Gulf Stream leaves the continental margin to enter the deep water regime; small amplitude propagating and growing meanders characterized the variability there. The Central Array measured velocity and temperature (as a proxy for density) at four levels in the water column, as well as the upper and deep level streamfunctions, all with mesoscale resolution. Near 70°W the path envelope exhibited a relative node, confined within a 40‐km band 55% of the time. Near 68°W the path envelope was over 3 times as wide, due to several elongated (“steep”) meander troughs and relatively steep meander crests. The crests typically propagated downstream without much growth. The troughs often stalled near 68°W, steepened, and persisted for one to several months. Two cases evolved into “S‐shaped” paths and subsequently formed rings. Even the time‐averaged fields showed a small trough in the mean path and thermocline structure. Whereas meanders of 20‐ to 60‐day periods had similar spectral levels throughout 70°–67°W, meanders with long periods (>85 day) accounted for the local minimum in variance at 70°W. Bottom pressure and velocity observations revealed repeated periods of intense (swirl speeds > 0.30 m s−1) abyssal eddies; the time‐averaged deep currents exhibited a mean cyclone centered 30 km offshore and downstream of the upper layer mean trough. The cross‐stream slope of the thermocline steepened linearly with path curvature, consistent with gradient wind balance. Structures are illustrated in the mapped fields consistent with baroclinic instability wherein troughs steepen and rings form.
Simulation of bomb radiocarbon in two global ocean general circulation models
Authors: Duffy P. B. , Eliason D. E. , Bourgeois A. J. , Covey C. C.
Published: February 2004

We perform simulations using global ocean general circulation models of the uptake by the ocean and the transport within the ocean of 14C from atmospheric nuclear bomb tests. The models used are (1) a modified version of the Geophysical Fluid Dynamics Laboratory modular ocean model (MOM) configured with simple horizontal subgrid‐scale mixing of tracers; (2) the same model using the Gent and McWilliams (1990) subgrid‐scale tracer transport parameterization, which includes isopycnal mixing; and (3) the ocean isopycnal (OPYC) model of J. M. Oberhuber, which uses isopycnal subgrid‐scale mixing of tracers within an isopycnal coordinate system. In addition, we perform another MOM simulation which illustrates the sensitivity of the results to the thickness of the surface mixed layer. To minimize differences in the results due to differences in forcings, all our simulations use the same formulations for surface fluxes of heat and fresh water and the same sea ice model. In addition, the numerical grids are all nearly the same. Comparison of our model results to revised estimates of oceanic bomb 14C from the Geochemical Ocean Sections Study observations and to annual mean temperature observations shows that our results are sensitive to the models' treatments of subgrid‐scale mixing of tracers and to the thickness of the surface mixed layer. Since the simulated vertical profiles of bomb 14C or temperature alone can be brought close to the observed profile by adjusting model parameters, we judge the realism of modeled vertical transport by how well vertical profiles of both bomb 14C and temperature can be simulated with one set of parameter values. By this standard, OPYC does better than our baseline configuration of MOM, which we attribute to the unrealistic treatment of subgrid‐scale mixing of tracers and of the surface mixed layer in this simulation. We illustrate this by showing that the use of the Gent and McWilliams tracer transport parameterization results in a markedly improved vertical temperature profile and almost no change in the bomb 14C distribution, compared with our baseline MOM simulation. Finally, we show that increasing the effective mixed layer thickness in MOM from 30 to 76 m results in a slightly more realistic vertical profile of bomb 14C but a slightly less realistic temperature profile.
Variational assimilation of simulated acoustic tomography data and point observations: A comparative study
Authors: Sheinbaum Julio
Published: February 2004

A simple advection‐diffusion model is used to generate synthetic acoustic tomography data and local temperature observations which are then assimilated by a numerical model. The data are assimilated using the following two methods: a variational adjoint method and a sequential assimilation technique formulated as a minimization problem. The “success” of the analyses are gauged by comparing them to the true fields and, in the adjoint assimilation experiments, by also performing analysis of error and resolution. Results show that with the same number of acoustic rays and point observations the initial temperature field is well reproduced whether acoustic data or local temperature data are used. Model and observation errors are introduced in the main assimilation experiments by adding random noise to the velocities of the model that generates the data but not into the assimilation model (model error). On top of that, extra random noise is added directly to the “observations” (observation error). The results are compared to experiments where only one source of error is introduced and to “identical twin” experiments in which there are neither model nor observation errors and therefore model and data are perfectly compatible. The effect of model error is to produce analyses whose error grows with time even in the presence of diffusion which tends to reduce such errors, whereas observation errors produce analyses similar to the identical twin experiments. The detrimental effect of model (and observation) errors can be reduced by weighting properly the background or a priori information which is used to regularize the analyses. It is found that the analyses from sequential assimilation methods are more sensitive to this weight. Adjoint‐variational assimilation is used to determine other model parameters (in this case, one of the advective velocities of the model) as a method to correct model deficiencies from data.
Open water production in Arctic sea ice: Satellite measurements and model parameterizations
Authors: Stern H. L. , Rothrock D. A. , Kwok R.
Published: February 2004

Sequential synthetic aperture radar (SAR) images from the ERS 1 spacecraft were routinely used as input to the Geophysical Processor System at the Alaska SAR Facility from 1992 through 1994 to produce sea ice displacement vectors on a 5‐km grid. We have combined some 5000 of these products, consisting of nearly 700,000 displacement vectors, into a set of 914 strips or mosaics of ice motion that span (mostly) 3‐day intervals. For each strip we compute the opening and closing of leads based on the change in area of the 5‐km grid cells. We also compute the area‐averaged deformation or strain invariants of the motion. The small‐scale (several kilometer) lead activity can be parameterized fairly well in terms of the large‐scale (several hundred kilometer) strain invariants. Sea ice models implicitly contain such a parameterization through their constitutive equations. The well‐known model of Hibler (1979) that uses a viscous‐plastic rheology and an elliptical yield curve is in good agreement with our data. The variance in the data about this theoretical relationship is less than that suggested by a random model of sea ice motion. The data also indicate that shearing deformation contributes to the opening and closing of leads. Models with two categories or levels of ice thickness generally do not take shearing deformation into account in the evolution of the ice concentration, nor do they redistribute ice as a result of ridging. We show how to add these features to two‐level models. However, we argue that three‐level models consisting of open water, thin ice, and thick ice are much better suited to the proper treatment of open water production, and we present such a model.
Adjoint assimilation of altimetric, surface drifter, and hydrographic data in a quasi‐geostrophic model of the Azores Current
Authors: Morrow Rosemary , De Mey Pierre
Published: February 2004

The flow characteristics in the region of the Azores Current are investigated by assimilating TOPEX/POSEIDON and ERS 1 altimeter data into the multilevel Harvard quasigeostrophic (QG) model with open boundaries (Miller et al., 1983) using an adjoint variational scheme (Moore, 1991). The study site lies in the path of the Azores Current, where a branch retroflects to the south in the vicinity of the Madeira Rise. The region was the site of an intensive field program in 1993, SEMAPHORE. We had two main aims in this adjoint assimilation project. The first was to see whether the adjoint method could be applied locally to optimize an initial guess field, derived from the continous assimilation of altimetry data using optimal interpolation (OI). The second aim was to assimilate a variety of different data sets and evaluate their importance in constraining our QG model. The adjoint assimilation of surface data was effective in optimizing the initial conditions from OI. After 20 iterations the cost function was generally reduced by 50–80%, depending on the chosen data constraints. The primary adjustment process was via the barotropic mode. Altimetry proved to be a good constraint on the variable flow field, in particular, for constraining the barotropic field. The excellent data quality of the TOPEX/POSEIDON (T/P) altimeter data provided smooth and reliable forcing; but for our mesoscale study in a region of long decorrelation timesO(30 days), the spatial coverage from the combined T/P and ERS 1 data sets was more important for constraining the solution and providing stable flow at all levels. Surface drifters provided an excellent constraint on both the barotropic and baroclinic model fields. More importantly, the drifters provided a reliable measure of the mean field. Hydrographic data were also applied as a constraint; in general, hydrography provided a weak but effective constraint on the vertical Rossby modes in the model. Finally, forecasts run over a 2‐month period indicate that the initial conditions optimized by the 20‐day adjoint assimilation provide more stable, longer‐term forecasts.
Inferring interannual changes in global upper ocean heat storage from TOPEX altimetry
Authors: White Warren B. , Tai Chang‐Kou
Published: February 2004

Good agreement is found between global patterns of TOPEX dynamic height and expendable bathythermograph (XBT) heat storage anomalies from approximately 30°S to 60°N for the 2‐year period from 1993 to 1994, with both variables dominated by the biennial signal. This is due to the physical dependence of absolute dynamic height upon relative steric height (i.e., an alias of heat storage). Regression finds these two variables correlated from 0.5 to 0.8 over the global ocean, with slopes of regression ranging from 0.05 to 0.15×109 W−s m−2 cm−1. While these statistics are not definitive, they do provide representative estimates of regression error, sampling error, and sampling frequencies for determining the role that TOPEX altimetry can play in estimating upper ocean heat storage over large portions of the global ocean. We examine this role within the framework of optimum interpolation methodology (Gandin, 1963), finding regression errors for inferred heat storage anomalies larger than instrumental errors expected from XBT heat storage estimates but of the same order as uncertainties incurred from subgrid ambient noise. While this increases the noise‐to‐signal ratio of the inferred heat storage anomalies by a factor of 2, the greater density of TOPEX observations reduces the interpolation errors for gridded fields of upper ocean heat storage anomalies to half those incurred from the smaller density of XBT sampling. So, when inferred heat storage anomalies are used to compute the time sequence in upper ocean heat storage anomaly over large portions of the global ocean during 1993–1994, errors of ±2 W m−2 are half those achieved with XBT sampling. Moreover, the time rate of change in upper ocean heat storage anomalies becomes statistically significant, with inferred heat storage anomalies in the North Atlantic Ocean remaining constant for 1993–1994 but increasing in both the Indian and Pacific Oceans. The latter dominate the time rate of change of upper ocean heat storage anomaly over the global ocean, yielding an average rate of 3 W m−2 over the 2‐year period.
A three‐dimensional numerical model of deep ventilation in temperate lakes
Authors: Walker Stephen J. , Watts Robert G.
Published: February 2004

Because of the variation in the temperature of maximum density with depth, it is known that the upper (above 250 m) and lower water columns circulate separately in deep temperate lakes. While near‐surface water overturns twice per year, the deep waters often mix only partially during a complete annual cycle. It is believed that deep convection is triggered by storm surge, forcing of some of the relatively cold upper water column downward through its compensation depth, so that it becomes unstable and sinks. The scale and intensity of the resulting deep water forming plumes are studied numerically. A high‐resolution model based on the nonhydrostatic Boussinesq equations is used. Deep water formation is initiated by applying a statically unstable, initial temperature profile over various regions of the domain at depths below 300 m. The grid spacing is small enough to resolve individual plumes which carry surface water to the lake bottom. The domain of study is also large enough that the geostrophic Eady (1949) wave breakup of newly formed deep water can be observed. It is found that when the initial instability is applied in midbasin, remote from the solid sidewalls, relatively little fresh deep water is formed and the fluid quickly reaches a geostrophic and hydrostatic balance. Vigorous vertical mixing ceases after 4 days, and the resulting baroclinic fluid has a well‐defined and predictable scale. In contrast, when the initial instability is applied immediately adjacent to a solid boundary, vigorous plume motion continues for the duration of the numerical simulations (12 days), producing a much greater volume of fresh deep water. A parametric study investigates the scale and intensity of this boundary mixing. It is found that reduced surface water temperature, Coriolis acceleration, or higher horizontal diffusion coefficients increase the rate of deep water renewal.
Barotropic tides in the global oceans from a nonlinear tidal model assimilating altimetric tides: 2. Altimetric and geophysical implications
Authors: Kantha Lakshmi H. , Tierney Craig , Lopez Joseph W. , Desai Shailen D. , Parke Michael E. , Drexler Laura
Published: February 2004

In this second part, we explore the implications of the tides derived from the high‐resolution, data‐assimilative, nonlinear barotropic global ocean tidal model described by Kantha (this issue) in altimetric analysis and geophysical applications. It is shown that when applied to the task of removing tidal sea surface height from TOPEX altimetric records, the model performance is comparable to other global tidal models in the open ocean as measured by the reduction in crossover variances. The performance is slightly better than that of the only other high‐resolution global tidal model from Grenoble (Le Provost et al., 1994). The results are however mixed in regions shallower than 1000 m and in semienclosed seas such as the Bering Sea, with the model performance slightly worse overall than the Grenoble model. Computations of total power input (and hence total tidal dissipation rate) are shown to be in excellent agreement with recent analyses of TOPEX data and geophysical observations. In addition, distributions of the tidal power input, tidal dissipation, and the power fluxes in the global oceans are shown for the two primary constituents, M2 and K1. Load tides in solid Earth due to ocean tidal loading fluctuations are also computed for the major semidiurnal and diurnal constituents. The load tides are shown to be large in the shallow seas adjacent to the coasts with high tides such as the Patagonian shelf, because of the higher resolution of this global tide model. This has potential implications in geophysical applications.
Assimilation of global versus local data sets into a regional model of the Gulf Stream system: 1. Data effectiveness
Authors: Malanotte‐Rizzoli Paola , Young Roberta E.
Published: February 2004

The primary objective of this paper is to assess the relative effectiveness of data sets with different space coverage and time resolution when they are assimilated into an ocean circulation model. We focus on obtaining realistic numerical simulations of the Gulf Stream system typically of the order of 3‐month duration by constructing a “synthetic” ocean simultaneously consistent with the model dynamics and the observations. The model used is the Semispectral Primitive Equation Model. The data sets are the “global” Optimal Thermal Interpolation Scheme (OTIS) 3 of the Fleet Numerical Oceanography Center providing temperature and salinity fields with global coverage and with bi‐weekly frequency, and the localized measurements, mostly of current velocities, from the central and eastern array moorings of the Synoptic Ocean Prediction (SYNOP) program, with daily frequency but with a very small spatial coverage. We use a suboptimal assimilation technique (“nudging”). Even though this technique has already been used in idealized data assimilation studies, to our knowledge this is the first study in which the effectiveness of nudging is tested by assimilating real observations of the interior temperature and salinity fields. This is also the first work in which a systematic assimilation is carried out of the localized, high‐quality SYNOP data sets in numerical experiments longer than 1–2 weeks, that is, not aimed to forecasting. We assimilate (1) the global OTIS 3 alone, (2) the local SYNOP observations alone, and (3) both OTIS 3 and SYNOP observations. We assess the success of the assimilations with quantitative measures of performance, both on the global and local scale. The results can be summarized as follows. The intermittent assimilation of the global OTIS 3 is necessary to keep the model “on track” over 3‐month simulations on the global scale. As OTIS 3 is assimilated at every model grid point, a “gentle” weight must be prescribed to it so as not to overconstrain the model. However, in these assimilations the predicted velocity fields over the SYNOP arrays are greatly in error. The continuous assimilation of the localized SYNOP data sets with a strong weight is necessary to obtain local realistic evolutions. Then assimilation of velocity measurements alone recovers the density structure over the array area. However, the spatial coverage of the SYNOP measurements is too small to constrain the model on the global scale. Thus the blending of both types of datasets is necessary in the assimilation as they constrain different time and space scales. Our choice of “gentle” nudging weight for the global OTIS 3 and “strong” weight for the local SYNOP data provides for realistic simulations of the Gulf Stream system, both globally and locally, on the 3‐ to 4‐month‐long timescale, the one governed by the Gulf Stream jet internal dynamics.