bluuebunny/crossref_metadata_2025 huggingface dataset:
Estimates of sea level in the tropical Atlantic Ocean using Geosat altimetry
Authors: Carton James A.
Published: February 2008

Monthly tropical Atlantic sea level is computed from Geosat altimetry for 1 year beginning in November 1986. The analysis is carried out using a collinear technique on a 1° × 1/3° grid. Monthly maps are presented, and the seasonal changes in the major currents are analyzed in the context of previous results from in situ data. Between 10° and 20°N the sea level shows a westward North Equatorial Current from July to April with the most persistent westward currents in the western part of the basin. Between 4° and 6°N an intense North Equatorial Countercurrent (NECC) exists throughout much of the year with geostrophic velocities exceeding 1 m/s. The NECC is intense in October‐November, but decays rapidly between December and January. In May the NECC returns, and intensifies uniformly in later months. The branch of the South Equatorial Current south of the equator prevails throughout most of the year west of 10°W. The strongest pressure gradient occurs between March and July. The zonal sea level slope along the equator reflects intensification of currents and large‐scale redistribution of water in response to seasonal winds. The seasonal change in zonal slope obtained here is similar to that estimated from in situ observations, but the changes are larger by 30%. Of the month‐to‐month variability of sea level, over half is described by the annual cycle. The strongest annual variability is in the countercurrent region, where the phase of the seasonal signal changes rapidly, along northwest Africa and the Gulf of Guinea. The latter two appear to be related to seasonal upwelling.
The relationship between the Monin‐Obukhov stability parameter and the bulk Richardson number at sea
Authors: Hsu S. A.
Published: February 2008

A generic formula, Z/L = κCTCD−3/2Rb, is derived in which Z/L is the Monin‐Obukhov stability parameter, κ is the von Kármán constant, CT and CD are the heat flux and drag coefficients, respectively, and Rb is the bulk Richardson number. Because CT and CD, and thus the ratio of Z/L and Rb, vary with wind speed and air‐sea temperature difference, a table is constructed based on the most recent modeling of CT and CD for operational reference. The table is consistent with results obtained from field experiments.
The structure of subtidal currents within and around Lydonia Canyon: Evidence for enhanced cross‐shelf fluctuations over the mouth of the canyon
Authors: Noble Marlene , Butman Bradford
Published: February 2008

Between October 1980 and April 1981, currents were measured within Lydonia Canyon and on the adjacent shelf and slope. The amplitude of the subtidal currents over the shelf and slope ranged between 10 and 30 cm s−1, but within the canyon, they were typically smaller than 5 cm s−1. The subtidal currents had well‐defined spatial structures over the shelf and the slope and in the middle and outer portions of the canyon. The along‐isobath flow over the shelf and slope was unaltered by the canyon. Currents within the canyon and just above it were driven up and down the canyon by the cross‐shelf pressure gradient in geostrophic equilibrium with the along‐shelf flow. The measurements suggest that the Coriolis force on the cross‐canyon flow, turbulent Reynolds stresses, and acceleration of the along‐canyon flow balanced the imposed pressure gradient for flow near the rim of Lydonia Canyon. The Coriolis force was not important in the deeper portions of the canyon, where baroclinic adjustments of the density field began to be an important factor in the momentum balance. A simple model indicates that the magnitude of the horizontal turbulent viscosity coefficient for subtidal flow in this narrow canyon is 106 to 107 cm2 s−1. The mixing indicated by the large amplitude of the viscosity coefficient was probably caused by the strong tidal currents present within Lydonia Canyon. On the shelf, along‐isobath currents were locally driven by the large‐scale component of the wind field; along‐shelf currents were equally correlated with local winds and with winds from sites 700 km northeast of Lydonia Canyon. Wind stress was not correlated with currents over the slope in water deeper than 450 m or with currents within the canyon.
Stochastically forced current fluctuations in vertical shear and over topography
Authors: Samelson R. M.
Published: February 2008

The effects of constant vertical shear of zonal velocity, uniform meridional bottom slope, and a deep meridional wall (representing the Mid‐Atlantic Ridge) on the response of a linear two‐layer quasi‐geostrophic ocean to stochastic wind stress curl forcing are investigated. The model is motivated by discrepancies in energy levels and distributions between recent observations and a similar model without shear or topography that predicted remote coherences relatively well. The results indicate that the presence of mean shear can significantly enhance the wind‐generation of surface‐intensified oceanic kinetic energy at frequencies above the zero‐shear cutoff for free baroclinic waves. The primary effect of the sloping topography is enhanced barotropic response near the barotropic topographic Sverdrup resonance, where topographic and planetary potential vorticity gradients cancel. The wall inhibits the barotropic response and increases coherences on the mooring side of the wall. Remote coherences for 49‐day period fluctuations are due to long barotropic waves as in the previous model, so the present model only partially reconciles the previous discrepancies.
A coupled ice‐ocean mixed layer model of the marginal ice zone responding to wind forcing
Authors: Ikeda Motoyoshi
Published: February 2008

A free‐drifting thermodynamic sea ice model is coupled with a continuously stratified ocean model, whose vertical mixing coefficients are determined by a turbulent closure scheme. The system has variability in the cross‐ice‐edge direction, while two‐dimensionality (along‐ice‐edge homogeneity) is assumed. The model is initially motionless and has no horizontal variability in ocean interior, whereas ice covers only a half portion. A uniform wind stress is imposed suddenly to drive the coupled system. The surface mixed layer is developed as time increases; i.e., a top few tens of meters are well mixed in a day. With an initial water temperature above a freezing point, the fresh mixed layer forms in the ice‐covered portion. A wind‐driven ice velocity is oriented to the right from the wind direction. A wind with the ice to the right (left) looking downstream produces upwelling (downwelling) under the ice edge caused by a difference in Ekman transport. An off‐ice (on‐ice) wind advects the ice toward the open water (ice‐covered) area, and makes the ice edge sharper (gentler) owing partially to faster movement in the region with higher ice concentration. In the off‐ice wind case, the mixed layer near the ice edge is shallower under development, so that a melt rate is smaller than that estimated by a prescribed‐depth mixed layer. An inertio‐internal gravity wave is generated by a transient variability in surface stress associated with a moving ice edge, most significantly in the case of the off‐ice wind, and enhances sharpness of the ice edge.
Blending sequential scanning multichannel microwave radiometer and buoy data into a sea ice model
Authors: Thomas D. R. , Rothrock D. A.
Published: February 2008

A method is presented for determining the concentrations of open water and of several ice types using multichannel satellite passive microwave data. The method uses the Kalman filter and provides the “best fit” to a time series of data. A crucial element of the procedure is a physical model of how the concentrations of ice types change with time in response to freezing, melting, aging of one ice type to another, and creation of open water by divergence of the ice cover. A measurement model relates the state of the ice cover to the multivariate microwave data. The procedure offers three distinct advantages over algorithms that interpret separately data from each instant in time: it provides a framework for incorporating additional data into the diagnosis of ice concentrations, it takes into account the known uncertainty in the microwave observations and the pure type signatures, and it allows the resolution of ice types with ambiguous signatures. Two examples are presented which make use of scanning multichannel microwave radiometer data and surface temperature and ice velocity data from drifting buoys to estimate the concentrations of open water, first‐year, second‐year, and older multiyear ice for a Lagrangian region of ice. Two other examples include melt ponds in place of second‐year ice. Some of the parameters in the physical model (melt rates) and in the measurement model (signature of second‐year ice or of frozen melt ponds) are unknown. Reasonable, but arbitrary, values of the unknown parameters are used in the examples.
A simple criterion for gyre formation by the surface outflow from a strait, with application to the Alboran Sea
Authors: Bormans Myriam , Garrett Chris
Published: February 2008

A survey of relevant numerical, laboratory, and observational studies, combined with our own laboratory experiments, suggests that the surface outflow from a strait generally forms a gyre in the adjacent sea if the exit corner is sharp. The gyre grows from an initial “separation bubble” of radius u/f, where u is the current speed and f the Coriolis frequency. The criterion for the maintenance of a coastal jet along a curved coast, instead of separation and gyre formation, seems to be that the Rossby number u(fRw)−1 < 1, with Rw the radius of curvature of the exit corner, so that the Coriolis force can hold the current to the coast. This synthesis disregards factors such as the variation of u across the current but, applied to the flow through the Strait of Gibraltar into the Alboran Sea, suggests that a gyre should always form as the exit corner is sharp. We suggest, though, that the relevant curvature is that where the interface between the two layers intersects the seafloor rather than where the sea surface meets the coast. If this is the case, supercritical flow or some subcritical flows at the eastern end of the Strait should still always lead to gyre formation, but the observed replacement of the gyre by a coastal jet along the Moroccan coast could be related to subcritical flow that is reduced below its normal value by a barotropic fluctuation or by changes in the interface depth in the Mediterranean.
Velocity and transport structure of the Kuroshio Extension at 35°N, 152°E
Authors: Hall Melinda M.
Published: February 2008

A current meter mooring maintained for 2 years at 35°N, 152°E as part of the “Wespac” array (Schmitz et al., 1987) sampled the Kuroshio Extension during the last 390 days of deployment. Based on a method developed by Hall and Bryden (1985) for the Gulf Stream at 68°W, velocity and temperature time series at five depths from 350 to 6000 m have been used to construct the representative horizontal velocity structure of the current. Cross‐stream position is quantified in terms of temperature at 350 dbar, and an alongstream flow direction is defined on the basis of the current shear. Integrated transport of the Kuroshio is 87±21 Sv, compared to 94±26 Sv for the Gulf Stream at 68°W based on analogous data sets and analysis techniques. The two currents are compared in terms of horizontal and vertical distributions of transport as well as relative strength of barotropic and baroclinic components. The Kuroshio has a larger relative barotropic component than the Gulf Stream, while the latter has a stronger, deeper thermocline expression. The effects of rotating velocity measurements to a continuously changing alongstream direction are examined for both currents; the major impact is to cut the apparent velocity variance in half, demonstrating that much of the original variance is due to the meandering of a quasi‐fixed velocity profile rather than the passage of eddies. Results from these somewhat unconventional analysis methods are placed in broader geographical context and compare favorably with more traditional methods of determining these currents' transports.
A case study of wave‐current interaction near an oceanic front
Authors: Liu A. K. , Jackson F. C. , Walsh E. J. , Peng C. Y.
Published: February 2008

High resolution directional wave spectrum data were obtained from two NASA airborne radars during the Frontal Air‐Sea Interaction Experiment (FASINEX) in February 1986. The observations show a significant change in the wave number spectrum across the front. On the basis of surveys from a towed sensor and on satellite imagery, the front location and current field are estimated. A numerical model is developed for the wave‐current interaction and is used to model the wave refraction across the frontal current. A parametric study is performed to demonstrate the effects of current meandering. The main consequence of meandering is the formation of caustics and shadow zone regions in which the wave energy is significantly enhanced or reduced. Spectral simulation along the aircraft track reveals a reduction of more than 60% in wave energy in the shadow zone; this is consistent with the observations.
Moored observations of current and temperature on the shelf and upper slope near ring 82B
Authors: Ramp Steven R.
Published: February 2008

An array of current meters was deployed along the continental shelf and upper slope from April 15 to July 1, 1982, as part of the Warm Core Rings Experiment. The array consisted of four mooring sites along the 325‐m isobath making an “X” with another line of moorings extending from the 60‐m to the 900‐m isobath. The purpose of the array was to examine the current and temperature variability induced at the shelf break and upper slope by the formation and passing of ring 82B. The velocity records consisted of 2‐ to 10‐day fluctuations embedded on a long‐period signal with a period ranging from 15 to 26 days, increasing offshore. The temperature records contained three time scales: The 2‐ to 10‐day scale, longer (about 1–2 months) scales due to movements of the shelf‐slope front induced by the ring, and an increasing trend due to seasonal warming. The 2‐ to 10‐day fluctuations of velocity and temperature were not statistically coherent with each other or with the weak surface wind stress but appear to be visually correlated with small (sub‐ring) scale undulations along the shelf‐slope front as seen in the advanced very high resolution radiometer (AVHRR) sea surface temperature imagery on an event by event basis. The observed periods and across‐shore scales of the long‐period motions were used with barotropic topographic Rossby wave (TRW) theory to estimate alongshore wavelengths of the order of 1000 km for these motions, westward and slightly offshore phase velocities, and onshore group velocity. Friction and wave refraction will not allow barotropic TRWs of the observed period to penetrate the upper slope unless the source is located nearby on the continental rise, which strongly suggests Gulf Stream warm core ring 82B as the energy source for the waves. The observed wave energy was used with the radiation model of Louis and Smith (1982) to estimate the rate at which energy was being lost from the source of the waves. The decay rate calculated from the moorings and the model was 0.24×10−13 J d−1, averaged over 6–14 weeks after ring formation, compared with 0.81×10−13 J d−1 for the decay rate of ring 82B (Olson et al., 1985) based on observed changes in the ring's available potential energy during April‐June 1982.
Frictional stress at a sheared double‐diffusive interface
Authors: Ruddick Barry R. , Griffiths Ross W. , Symonds Graham
Published: February 2008

We describe the results from a laboratory experiment designed to directly measure the stress exerted across a sheared interface which lies between two convecting layers. The two‐layer convection was set up in an annular tank which was rotating slowly so as to provide a basic “store” of angular momentum. Slow radial pumping was used in each layer to drive azimuthal flows, and measurement of the layer velocities provided near‐direct measurements of azimuthal stress. The stress was found to be consistent at all times with that due to molecular viscosity and was independent of the buoyancy flux associated with the convection. The uncertainties of the experimental observations, taken together with the hypothesized stress‐shear relation of Ruddick (1985; equation (1) of this paper), imply a maximum turbulent Prandtl number (absolute value of the ratio of momentum and density diffusivities) of O(1). The experimental results of Ruddick (1985) are reconsidered, and it was concluded that the dependence of internal wave decay rate of buoyancy flux observed in those experiments is a result of the large eddy viscosity in the turbulent mixed layers acting on the strain field of the wave.
Sea ice and oceanic processes on the Ross Sea continental shelf
Authors: Jacobs S. S. , Comiso J. C.
Published: February 2008

We have investigated the spatial and temporal variability of Antarctic sea ice concentrations on the Ross Sea continental shelf, in relation to oceanic and atmospheric forcing. Sea ice data were derived from Nimbus 7 scanning multichannel microwave radiometer (SMMR) brightness temperatures from 1979–1986. Ice cover over the shelf was persistently lower than above the adjacent deep ocean, averaging 86% during winter with little month‐to‐month or interannual variability. The large spring Ross Sea polynya on the western shelf results in a longer period of summer insolation, greater surface layer heat storage, and later ice formation in that region the following autumn. Newly identified Pennell and Ross Passage polynyas near the continental shelf break appear to be maintained in part by divergence above a submarine bank and by upwelling of warmer water near the slope front. Warmer subsurface water enters the shelf region year‐round and will retard ice growth and enhance heat flux to the atmosphere when entrained in the strong winter vertical circulation. Temperatures at 125‐m depth on a mooring near the Ross Ice Shelf during July 1984 averaged 0.15°C above freezing, sufficient to support a vertical heat flux above 100 W/m2. Monthly average subsurface ocean temperatures along the Ross Ice Shelf lag the air temperature cycle and begin to rise several weeks before spring ice breakout. The coarse SMMR resolution and dynamic ice shelf coastlines can compromise the use of microwave sea ice data near continental boundaries.
Advective surface velocities derived from sequential images for rotational flow field: Limitations and applications of maximum cross‐correlation method with rotational registration
Authors: Kamachi M.
Published: February 2008

An objective technique is developed to calculate advective surface velocities from sequential images. A pattern matching method is used with the identification of maximum cross correlation (MCC) between a template window in the first image and search areas in the second image. We examined the limitations of the MCC method in two cases: (1) eddy size (LE) much smaller than the radius of deformation (LD), and (2) LE≳LD. In the first case an eddy is regarded as a particle. Maximum detectable time period is estimated to be about 1 day. For the second case we developed the MCC method to detect a rotational motion. We compared velocity fields derived from the method with a real velocity field in a numerical analysis of tracer and quasi‐geostrophic eddy fields. The optimal template size is about the eddy diameter which spans between the maximum velocity points. We also applied the MCC method to infrared images in the Kuroshio‐Oyashio confluence zone.
The Western Mediterranean Circulation Experiment (WMCE): Introduction
Authors: La Violette Paul E.
Published: February 2008

The purpose of the Western Mediterranean Circulation Experiment was to study the circulation of the western Mediterranean Sea from the Strait of Sicily to the Strait of Gibraltar using scales ranging from basin‐size to 1 km and depths from the surface to the deepest layers. An international consortium of multidisciplined marine investigators formed its study group; each had programs adapted to achieve the common goal: to derive the spatial and temporal variability of the western Mediterranean Sea circulation.

WMCE was a logical outgrowth of a number of field campaigns and data analyses (see, for example, Donde Va? Group [1984]) that raised as many questions as answers. The WMCE Consortium was formed during a 1983 NATO (North Atlantic Treaty Organization) study meeting in La Spezia, Italy, when the time and resources were judged appropriate for a study of the western Mediterranean as a single entity. Such a study would answer the following questions:

1. What are the major features of the western Mediterranean circulation, and how do they vary in space and time?

2. What are the physical forcing mechanisms?

3. How can this knowledge be implemented into numerical models?

4. How does the circulation affect the chemical, biological, and optical properties of the western Mediterranean Sea?
Sensitivity and uncertainty analysis of a short‐term sea ice motion model
Authors: Thomson N. R. , Sykes J. F.
Published: February 2008

In the modelling of sea ice motion, a major concern focuses on how sensitive model simulation results are to changes in the model inputs or the system parameters. Parallel to this concern is the manner in which the uncertainty in the model inputs or system parameters manifests itself as uncertainty in model results. In this paper, the theory required for the sensitivity and uncertainty analysis of a short term ice motion model is presented. The governing equations for this ice motion model are composed of the standard equation of motion in conjunction with a two‐level thickness distribution model. To ascertain model sensitivities, a method based on the adjoint operator technique is utilized. Uncertainties in the model simulation results are determined by a first‐order, second‐moment method. Two formulations of the adjoint state equations are presented, one involving the governing partial differential equations (continuous formulation) and the other involving the discrete or numerical form of the governing equation (discrete formulation). The resulting sensitivity and uncertainty methodology is applied to a 4‐day ice motion episode in the southern Beaufort Sea. The sensitivity and uncertainty of the calculated ice velocity and the ice thickness at a particular location within the domain are discussed.
Precision orbit determination for the Geosat Exact Repeat Mission
Authors: Shum C. K. , Yuan D. N. , Ries J. C. , Smith J. C. , Schutz B. E. , Tapley B. D.
Published: February 2008

Precise ephemerides have been determined for the Navy's Geodesy Satellite (Geosat) Exact Repeat Mission (ERM) using an improved gravity field model, PTGF‐4A (Shum et al., 1989). The Geosat orbits were computed in a terrestrial reference system which is tied to the reference system defined by satellite laser ranging (SLR) to LAGEOS through a survey between the TRANET Doppier receiver and the SLR system located at Wettzell, Federal Republic of Germany. The remaining Doppler tracking station coordinates were estimated simultaneously with the geopotential in the PTGF‐4A solution. In this analysis, three continuous 17‐day Geosat orbits, which were computed using the 46‐station TRANET data and global altimeter crossover data, have a crossover residual root‐mean‐square (rms) of 20 cm, indicating that the Geosat radial orbit error is of the order of 20 cm. The orbits computed based on data collected by a 7‐station OPNET tracking network and crossover data have the same level of accuracy. Collinear sea surface height analysis using the improved orbits for two Geosat 17‐day repeat cycles yields an rms height difference of 16 cm. Spectral analysis of the sea surface height difference indicates that the long‐wavelength orbit error has been reduced significantly. Finally, the Geosat altimeter time tag bias, based on 54 days of the Geosat ERM altimeter crossover data, is 5±3 ms.
Ventilation rates of the waters in the Nansen Basin of the Arctic Ocean derived from a multitracer approach
Authors: Schlosser Peter , Bönisch Gerhard , Kromer Bernd , Münnich Karl Otto , Koltermann Klaus Peter
Published: February 2008

Tritium/3He−, 14C− and helium/neon data from a station located in the central Nansen Basin of the Arctic Ocean are reported and discussed. It is demonstrated that 3He is a valuable tracer for studies of the upper water column and, together with tritium, 3He provides a rough time scale of the ventilation of these waters. The apparent tritium/3He age is about 7 years in the surface water and about 14 years in the lower part of the halocline. Water of Atlantic origin defined by a temperature maximum shows tritium/3He ages between 7 and 9 years. This variation of ages indicates that the circulation of the individual water layers of the upper water column is largely decoupled and occurs on different time scales. Tritium/3He ages observed in the Arctic Intermediate Water suggest a ventilation on a time scale of 15 to 20 years (depth <1000 m). The mean age of the Eurasian Basin Bottom Water is estimated on the basis of 14C data to approximately 200 to 250 years. There is no indication of noble gas fractionation due to sea ice formation below the surface layer. From analysis of the 3He background it appears that there is a small primordial 3He component in Eurasian Basin Bottom Water.
Constraining the north Atlantic circulation with tritium data
Authors: Mémery Laurent , Wunsch Carl
Published: February 2008

The circulation of the north Atlantic as deduced from an inverse calculation is tested against the historical record of tritium. The circulation model is first used to do a “forward” calculation of the tritium transient using published estimates of atmospheric injection rates, and of plausible estimates of the tracer history at the open boundaries of the model. Major discrepancies are found between this calculation and the observations of interior distributions of tritium. An attempt is then made to remove these discrepancies by modifying the atmospheric injection rates and the initial estimates of open boundary time histories, treating these boundary conditions as control variables. The procedure requires answering difficult questions as to the accuracy with which atmospheric transfers to the ocean and the boundary conditions at the northern limits of the model are known. Although removal of the model and observational tritium estimate differences is not completely satisfactory, it appears that existing tritium observations would only weakly constrain a more realistic circulation inversion. The computation, however, does suggest a strategy for future use of transient tracers for constraining circulation models.
Internal waves under the Arctic pack ice during the Arctic Internal Wave Experiment: The coherence structure
Authors: Levine Murray D.
Published: February 2008

The spectral composition of internal gravity waves under the Arctic pack ice during the Arctic Internal Wave Experiment (AIWEX) was found to be strikingly different from observations at lower latitudes. Time series of vertical displacement were inferred from horizontal and vertical arrays of temperature and conductivity sensors. Frequency spectra indicate a whiter spectrum (spectral slope near −1) and a less energetic wave field (by a factor of 0.02) than observations at lower latitude. The analysis of vertical and horizontal coherences revealed a horizontally isotropic wave field that is consistent with assumptions of a random field of linear internal waves. The wavenumber bandwidth of the wave field is about a factor of 10 wider than found at lower latitude.
A model of sea ice and the upper ocean mixed layer off Labrador
Authors: Yao T. , Ikeda M.
Published: February 2008

An existing dynamic, thermodynamic ice model is coupled to an existing bulk mixed layer model of the upper ocean. The model is applied to the seasonal evolution of ice in the marginal ice zone off Labrador. Ocean currents are prescribed. Daily atmospheric observations are used as forcing. The coupled model simulates the growth phase of ice and the evolution of the upper ocean temperature and salinity reasonably well. The results show that mixed layer deepening can account for the offshore ice extent. The southern extent of ice is influenced more by the meridional gradients in atmospheric heat flux and in ocean heat content. Lateral advection within the ocean is important in determining the southern ice extent.
Environmental effects on acoustic measures of global ocean warming
Authors: Semtner Albert J. , Chervin Robert M.
Published: February 2008

Munk and Forbes (1989) have proposed an acoustic technique for measuring the ocean warming caused by the buildup of greenhouse gases in the atmosphere. Travel time from a source in the South Indian Ocean will be monitored at receivers as far away as the North Atlantic and North Pacific. However, there are natural perturbations of acoustic travel time over long distances as a result of oceanic mesoscale eddies, seasonal fluctuations, and interannual variability. Results from a global eddy‐resolving ocean model are used here to assess the importance of two of these noise factors. Neither mesoscale nor seasonal effects are found to be large enough to obscure the anticipated signal of global change in the ocean. Analyses of the modeled temperature trends and variability along three selected paths give insights into where mesoscale and seasonal variability affect the acoustics.
Observations of Gulf Stream ring 83‐E and their interpretation using feature models
Authors: Glenn Scott M. , Forristall George Z. , Cornillon Peter , Milkowski George
Published: February 2008

Analysis techniques were developed to track and estimate the propagation velocity, size, shape, orientation, and swirl velocity of Gulf Stream warm core rings. The methods involve fitting idealized warm core ring feature models to observed surface fronts in the satellite imagery, to expendable bathythermograph survey data, and to satellite‐tracked drifting buoy data. The ring feature models are analytic structure functions with adjustable parameters. In this case the ring is modeled as an isolated translating elliptic paraboloid with a swirl velocity that increases linearly with distance from the center. Use of the feature models is illustrated with data collected from warm core ring 83‐E. Gradient currents based on the bathythermograph survey of ring 83‐E and the feature model agree well with currents measured by expendable current profilers. A comparison of the time series of ring 83‐E feature model parameters indicates that not only is the feature model analysis of the buoy track an effective method for tracking and monitoring the evolution of Gulf Stream rings, it also provides useful forecasts of ring locations based simply on a persisted propagation velocity. These techniques were used to provide real time support for exploratory deepwater drilling operations off the U.S. east coast.
Orographically trapped coastal wind events in the Pacific Northwest and their oceanic response
Authors: Hermann Albert J. , Hickey Barbara M. , Mass Clifford F. , Albright Mark D.
Published: February 2008

Three ageostrophic, Orographically trapped wind events in the coastal zone of the Pacific Northwest are examined. Time series of wind speed, wind direction, air temperature, sea level pressure, ocean currents, and ocean temperature are utilized to establish the general features of these events and to characterize the oceanic response. It is suggested that these atmospheric events are highly nonlinear bores or gravity currents, rather than quasi‐linear Kelvin waves. Observed oceanic responses include a drop in sea surface temperature (SST) of 1°–3°C offshore to approximately 150 km (approximately the atmospheric Rossby radius) and a rise in SST of similar magnitude within ∼15 km of the shore. A set of reduced‐gravity, fully nonlinear single layer models is utilized to investigate the dynamics of the atmospheric features and their oceanic response, and a possible air‐sea interaction. The models reproduce several of the first‐order features of the air and sea phenomena, such as the development of the atmospheric front, and the wake of near‐inertial waves in the ocean.
Implications of the nonlinear equation of state for upwelling in the ocean interior
Authors: McDougall Trevor J. , You Yuzhu
Published: February 2008

The nonlinear nature of the in situ density of seawater as a function of the potential temperature, pressure and salinity causes two vertical advection processes (thermobaricity and cabbeling) and also complicates the use of microstructure data to deduce upwelling velocities. Cabbeling and thermobaricity are evaluated and mapped on some neutral surfaces in each of the world's oceans by taking the lateral flux of scalars to be parameterized by a lateral diffusivity. In most of the ocean, these two processes are weak, but where there is a significant epineutral gradient of potential temperature, the downwelling due to cabbeling is quite large. In the Southern ocean, where there is a large slope of the neutral surfaces, thermobaricity causes a larger downwelling velocity than cabbeling, and the two processes together cause a dianeutral velocity of about −2×10−7 m s−1. The complementary roles of vertical mixing and vertical advection in achieving water‐mass conversion are demonstrated, since maps of the dianeutral motion caused by vertical mixing are quite different to maps of water‐mass conversion caused by the same process. This emphasizes the need to include both vertical advection and vertical mixing in ocean models. The method that is used to infer the upwelling velocity from microstructure dissipation measurements is also significantly affected by the nonlinear nature of the equation of state. The extra term that needs to be included in this method is a strong function of depth, changing sign at a depth of about 1500m.
A method for optimal analysis of fields with spatially variable mean
Authors: Le Traon P. Y.
Published: February 2008

This brief report addresses the problem of objective analysis of fields with spatially variable mean. Conventional objective analysis is confined to fluctuations, where the larger scale is assumed to be known; this report shows how to use the method described by Davis (1985) to estimate simultaneously a large‐scale component and a mesoscale component with an accurate error budget. The extension to multivariate analysis is also discussed. The method is then applied to simulated data; this illustrates its contribution in relation to conventional objective analysis.
Cross sections and modulation transfer functions at L and Ku bands measured during the Tower Ocean Wave and Radar Dependence Experiment
Authors: Keller William C. , Plant William J.
Published: February 2008

Normalized radar cross sections and modulation transfer functions (MTFs) for microwave backscattering from the sea surface have been measured at both L‐ and Ku‐ Bands during the Tower Ocean Wave and Radar Dependence Experiment. Long waves during the experiment were usually not generated by the local wind, so a unique opportunity was afforded to investigate the effects of arbitrary wind, wave, and antenna angles on the backscatter. Data were collected using horizontal polarization at L‐ Band and vertical polarization at Ku‐ Band. Cross sections at L‐ Band are shown to be isotropic with respect to wind‐antenna angle and nearly independent of wind speed except at the lowest wind speeds. Ku‐ Band cross sections, on the other hand, show the expected wind‐antenna angle anisotropy and wind speed dependence. The Ku‐ Band cross sections agree well in magnitude with previous wave‐tank and satellite measurements, casting doubt on the dependence of cross section on antenna height which has been suggested in the literature. At both frequencies the data suggest that cross sections may be lowered slightly when long waves propagate at large angles to the wind. Consistent with the cross section results, MTFs at L‐ Band are shown to depend little on wind speed, while those at Ku‐ Band decrease with wind speed. These results agree with previous studies. At both frequencies, MTFs depend little on wind‐antenna angle for a constant wave‐antenna angle. On the other hand, for a constant wind‐antenna angle, MTFs decrease with increasing wave‐antenna angle. Implications of these results for radar imagery of the sea surface and for the theoretical interpretation of the origin of MTFs are discussed.
Variations of global mesoscale eddy energy observed from Geosat
Authors: Shum C. K. , Werner R. A. , Sandwell D. T. , Zhang B. H. , Nerem R. S. , Tapley B. D.
Published: February 2008

The global distribution of eddy kinetic energy has been synoptically observed from analysis of the Geosat Exact Repeat Mission (ERM) altimeter data collected for a 2‐year period from November 1986 through November 1988. Using a technique developed by Sandwell and Zhang (1989), altimeter data from forty‐four 17‐day repeat cycles (2 years) were processed into sea surface slopes along the satellite ground track, averaged, and filtered to produce a mean sea surface slope profile having an estimated accuracy of 0.2 μrad (2 cm sea level change over 100 km distance). A series of global eddy kinetic energy maps, each averaged over 3 months, and their mean were then generated. The maximum mean eddy kinetic energy per unit mass exceeds 2000 cm2/s2 for most of the western boundary currents; however, it only reaches approximately 500 cm2/s2 for the Antarctic Circumpolar Current (ACC). More than 65% of the world ocean has relatively low variability with an eddy kinetic energy of less than 300 cm2/s2. Results obtained from this study are in general agreement with other Geosat ocean variability studies (e.g., Zlotnicki et al., 1989). However, significantly higher variability is found when compared with either Seasat or ship drift data. Significant seasonal variations were found in the Gulf Stream and Kuroshio currents. The ACC system exhibits no apparent seasonal variation.
Observing the tropical Atlantic Ocean in 1986–1987 from altimetry
Authors: Arnault Sabine , Menard Yves , Merle Jacques
Published: February 2008

Geosat altimeter data from November 1986 through November 1987 are analyzed over the tropical Atlantic Ocean, between 20°N to 20°S and 60°W to 20°E, using the colinear profile method. The altimetric sea level anomalies are consistent with the large‐scale signal of the tropical Atlantic as observed by historical dynamic height. The agreement in the North Equatorial Countercurrent region (around 5°N) is particularly good. Comparison of altimetric anomalies with 1986–1987 in situ data points out the part of interannual events in the explanation of the differences encountered between altimetry and climatology. Altimetric data are also compared with the results of a simple model forced with monthly 1986–1987 wind stresses. This comparison confirms and emphasizes the hypothesis of a weak and early 1987 summer up welling in the Gulf of Guinea.
Estimation of the processes controlling variability in phytoplankton pigment distributions on the southeastern U.S. continental shelf
Authors: McClain Charles R. , Ishizaka Joji , Hofmann Eileen E.
Published: February 2008

A series of five CZCS images from the southeastern U.S. continental shelf for April 10–16, 1980, were combined with concurrent moored current meter measurements to estimate the processes controlling the observed variability in chlorophyll concentration and distribution in this region. The approach used for this analysis is a series of calculations with the equation governing the space and time distribution of a nonconservative quantity such as chlorophyll. The CZCS chlorophyll distributions and current meter measurements allow estimation of the terms in the governing equation which gives an indication of the relative contribution of advective, diffusive, and local processes in producing the observed chlorophyll distributions on the southeastern U.S. continental shelf. The estimates of the terms in the governing equation from observations show that advective, diffusive, and local processes all contribute to the phytoplankton distributions and that the relative contribution of each changes with time and location. For the 5 days considered in this analysis, the order of importance of processes controlling variability in phytoplankton distributions determined from observations was biologicalvertical processes (36%), diffusion (33%), advection (16%), and local time change (15%). As a comparison, a similar series of calculations were performed with simulated chlorophyll distributions obtained from a numerical physical‐biological model constructed for the southeastern U.S. continental shelf ecosystem that used circulation fields derived from an optimal interpolation of the current meter observations. The ranking of the importance of processes obtained from the simulated distributions is; advection (32%), biological‐vertical processes (27%), diffusion (20%), and local time change (21%). Thus the two approaches provide different estimates of the processes controlling phytoplankton variability on the southeastern U.S. continental shelf. The most likely cause of the different results obtained with the two methods is error introduced into the calculations by the transient nature and sharp gradients associated with Gulf Stream frontal eddies. In spite of these results, the approach described in this analysis may have promise for computing chlorophyll budgets from ocean color measurements obtained in less dynamic environments.
Baroclinic semidiurnal tidal currents over the continental shelf off northern California
Authors: Rosenfeld Leslie K.
Published: February 2008

Time‐varying depth‐dependent currents of semidiurnal frequency are identified in data collected over the northern California continental shelf during the 1982 up welling season as part of the Coastal Ocean Dynamics Experiment. An increase in semidiurnal band kinetic energy is seen during wind relaxation periods when stratification over the shelf and upper slope increases. Comparison with internal wave theory shows the vertical and horizontal structure of the semidiurnal band horizontal current and temperature fluctuations to be consistent with a first baroclinic mode internal wave with a horizontal wavelength of 20–30 km. Over the continental shelf the semidiurnal band temperature fluctuations show a phase shift which may be evidence of the influence of the subcritical bottom slope and/or the effects of bottom friction. The bottom slope is supercritical over the upper continental slope, suggesting that generation of the semidiurnal internal waves observed over the shelf may take place near the shelf break. Application of Baines' (1982) theory for the generation of internal tides underpredicts the observed baroclinic tidal currents.
Currents at Lindenkohl Sill in the southern Gulf of Maine
Authors: Brooks David A.
Published: February 2008

From June 22 to September 3, 1986, current speeds and directions were measured at depths of 76, 127 and 212 m over Lindenkohl Sill (225 m), which controls an important deepwater spreading path from Georges Basin into Wilkinson Basin in the Gulf of Maine. Most (70%) of the low‐frequency (subtidal) variability occurred in a barotropiclike mode in which the inward currents were coherent with alongshore wind stress for ∼5‐day periods early in the record and for ∼3‐day periods late in the record; the coherence weakened during the central part of the record when dense Atlantic slope water from Georges Basin moved over the mooring site. Most of the remaining variance appeared in a baroclinic mode that added constructively to produce surprisingly large (∼1 m s−1) near‐bottom currents during the wind‐coherent intervals; at other times the modes interfered to produce near‐zero currents over the sill while middepth and upper level currents were intensified. The relationship between the wind stress and the currents suggests the action of a density‐modulated coastal upwelling mechanism in which the deep inward currents over the sill respond directly to northeastward alongshore wind stress at times of weak stratification; increased density stratification caused by water mass adjustments or seasonal heating produces a more complex baroclinic response. It is suggested that the springtime onset of northeastward winds over the Gulf of Maine may play an indirect but important role in the yearly intensification of the baroclinic circulation in the gulf by controlling or modulating the indraft of slope water into the inner basins.
An assessment of possible ozone‐solar cycle relationship inferred from NIMBUS 4 BUV data
Authors: Chandra Sushil
Published: February 2008

The total ozone and the ozone mixing ratio at various pressure levels in the stratosphere measured from the NIMBUS 4 backscattered ultraviolet (BUV) experiment over a 7‐year period (1970–1977) comprise a very comprehensive data base available to study the possible effects of solar variability on stratospheric ozone. It is shown that with the decrease in solar activity from 1970 to 1976, the globally averaged ozone inferred from NIMBUS 4 data decreases from about 10–12% in the upper stratosphere to about 1–3% in the lower stratosphere. This systematic decrease at all pressure levels is modulated by a quasi‐periodic oscillation of about a 2‐year period. Both the systematic decrease and the quasi‐periodic oscillations in ozone seem to be correlated with the conventional indices of solar activity suggesting a solar UV‐ozone relationship. Despite this apparent correlation, it is difficult to account for the observed changes at various pressure levels with our current understanding of the photochemical models and the solar UV flux variations over a solar cycle. The agreement between the observed and the calculated profiles of ozone mixing ratio is considerably improved if the BUV data are corrected for the changes in the instrument sensitivity using Umkehr observations as a reference.
Sulfate accumulation in a sea breeze/land breeze circulation system
Authors: Cass Glen R. , Shair Fredrick H.
Published: February 2008

An atmospheric tracer study using SF6 was conducted on July 22, 1977, to examine the origin of the high particulate sulfate concentrations observed in coastal Los Angeles County. It was found that the sea breeze/land breeze circulation system in the Los Angeles Basin both increases the retention time for sulfate formation in the marine environment and causes individual air parcels to make multiple passes over large coastal emissions sources. Day‐old sulfur oxides emissions advected out to sea by the land breeze at night were estimated to be the largest single contributor to 24‐hour average sulfate air quality over land the next day. In contrast, 24‐hour average SO2 concentrations were dominated by fresh emissions from nearby sources. The overall rate of SO2 transformation to form particulate sulfur oxides along some trajectories that spent a considerable time over the ocean at night probably exceeds the rate that can be explained by known photochemical processes acting during the daylight portion of these trajectories. This suggests that appreciable aerosol formation may occur in a polluted marine environment at night.