bluuebunny/crossref_metadata_2025 huggingface dataset:
Response of Subarctic Pacific diatom fluxes to the 1982–1983 El Niño disturbance
Authors: Takahashi Kozo
Published: February 2008

Time series sediment trap samples were collected during October 1983 to August 1984 (second year) at 3800 m at station PAPA (50°N, 145° W; water depth 4200 m) in the northeastern North Pacific. Components of diatom fluxes have been measured and compiled with previously published 1982–1983 records (first year). Most diatom species showed significantly lower cumulative fluxes in the second year than in the first. Second‐year seasonal flux patterns were also notably different from those in the first year. Many of the differences are attributed to the second year's hydrographic change in density structure in the upper 100 m, resulting in a poor nutrient supply to the euphotic layer. The changes in the density structure were coincidental with the 1982–1983 El Niño disturbance. Such an observed flux decrease was analogous to notable changes in hydrography and biota reported elsewhere in the eastern subarctic Pacific which were reported to be due to the El Niño. Diatom fluxes clearly responded to this unusual climate change as well as to normal seasonal cycles. The valve percentages of each species within total centric diatoms showed monotonous values throughout year 2, when productivity was generally low. The percent values were similar to those of year 1's low production period, consistently indicating that these are low productivity signals. High diatom fluxes were accompanied by high percentages of Chaetoceros, suggesting high productivity signals. This strongly supports the contention that diatoms are sensitive climatic and productivity indicators. Maximum diatom carbon flux is mainly contributed by two taxa, Denticulopsis seminae, the smallest sized and most numerous taxon, and Rhizosolenia styliformis, the largest sized taxon. A sizable contribution of carbon flux by R. alata f. gracillima was observed in year 1, but this was not repeated in year 2 due to the El Niño disturbance.
Marine boundary layer modification across the edge of the Agulhas Current
Authors: Jury Mark , Walker Nan
Published: February 2008

Aerial survey results are presented which show a modification of the lower atmosphere on crossing the edge of the Agulhas Current near 37°S, 20°E. Profiles of air temperature and dewpoint show increases in marine boundary layer heights of about 400 m as sea surface temperatures rise suddenly from 18° to 24°C. Westerly winds increase across the NE‐SW aligned sea surface temperature front by 7 m s−1, in a sea breeze‐like circulation. Surface latent and sensible heat fluxes to the atmosphere change from 100 W m−2 to 340 W m−2 on entering the Agulhas Current because of feedback processes between warmer sea surface temperatures, turbulence, and wind stress. The data sample is small and statistically limited, but the case study results indicate that the response of the lower atmosphere to the Agulhas Current is similar to that found over the Gulf Stream and other warm western boundary currents.
Inferring the nature of microstructure signals
Authors: Marmorino G. O. , Greenewalt D.
Published: February 2008

Further study is done of Holloway and Gargett's (1987) suggestion that it is possible to distinguish salt‐fingering microstructure signals from those induced by turbulence by using the kurtosis (the relative flatness) of the scalar probability distribution. Support for their result comes from an examination of towed measurements of conductivity gradient fluctuations from turbulent and salt finger patches in the Sargasso Sea; however, when data are examined from presumably salt finger interfaces in the thermohaline staircase east of Barbados (the Caribbean Sheets and Layers Transects (C‐SALT) experimental region), data segments are found having both the suppressed kurtosis values expected of salt fingering and the high values associated with turbulence. It is concluded that either turbulence or some other chaotic process is competing with salt fingering in the C‐SALT data or that the kurtosis discriminant is not as useful as Holloway and Gargett suggest.
On formulating two‐dimensional vertically integrated hydrodynamic numerical models with an enhanced representation of bed stress
Authors: Davies Alan M.
Published: February 2008

A convolution method is developed for computing the bottom stress in a vertically integrated hydrodynamic numerical model. By this means the bottom stress is no longer related to the depth mean current as in a “conventional” vertically integrated model, and an enhanced representation of the bottom stress is possible (an “enhanced” vertically integrated model). Consequently, the enhanced model can reproduce phase lags and differences in direction between bed stress and depth mean current, which cannot be taken into account in conventional vertically integrated models. A linear or quadratic frictional formulation can be used with the method. Idealized calculations of wind‐induced flow in a rectangular basin show that the bed stress computed by the convolution method is in excellent agreement with that derived using a full three‐dimensional model. However, significant differences in the bed stress between the three‐dimensional and conventional two‐dimensional model are evident. Also in this paper, a method is developed for extracting current profiles from vertically integrated models. Current profiles extracted from the enhanced two‐dimensional model, for a range of frictional effects, are found to be in good agreement with those computed using a full three‐dimensional model.
A method for determining equivalent depths of the atmospheric boundary layer over the oceans
Authors: Yu Tsann‐Wang
Published: February 2008

The concept of an equivalent depth of the atmospheric boundary layer is discussed in the framework of vertically integrated boundary layer equations. A method is presented by which these depths may be computed from sea level pressure analyses and ocean surface wind speed measurements. An explicit representation that yields a realistic value for the equivalent depth from the equator to the pole is given in terms of these parameters. For this study, the equivalent depths were calculated from 3 days of altimeter wind speed data taken from Seasat. This study shows that over the mid‐latitude and polar regions and in the range of surface wind speeds between 5 m/s and 15 m/s, the equivalent depths are calculated to be about 700–800 m, comparable to the heights of the atmospheric boundary layer; in the polar region, both scale heights are also quite comparable in values, ranging between 200 and 600 m; over the tropics, however, the conventional scale heights, as is well known, are unreasonably large, whereas the calculated equivalent depths are quite realistic and well defined. It is therefore concluded that the method presented in this study may be used to infer heights of the atmospheric boundary layer over the global oceans. In addition, the calculated equivalent depths together with the measured surface wind speeds provide an estimate of the spin up time for the boundary layer flows. During the 3‐day period, the spin up times are found to vary between 4.5 hours in the polar regions to about 39.4 hours in the mid‐latitudes.
An intercomparison of four mixed layer models in a shallow inland sea
Authors: McCormick Michael J. , Meadows Guy A.
Published: February 2008

Four mixed layer (ML) models after Denman [1973] (KLD), Garwood [1977] (RWG), McCormick and Scavia [1981] (K), and Thompson [1976] (RT) were compared against an extensive water temperature data set collected in the central basin of Lake Erie during the summer of 1979. Results suggest that all four models are nearly equal in their ability to satisfactorily simulate surface water temperatures. However, if diurnal physical processes are of interest, then the model ability to simulate both the ML depth and the energy level associated with entrainment dh/dt becomes crucial. While three of the models, KLD, RT, and RWG, were satisfactory in simulating the ML depth, only two of the models RT and RWG, were satisfactory in matching the energy levels seen in the entrainment spectra of dh/dt. This agreement suggests that the shear velocity ΔV entrainment scaling plays a critical role in the cycling of shallow depth mixed layers.
Short‐term ice motion modeling with application to the Beaufort Sea
Authors: Thomson N. R. , Sykes J. F. , McKenna R. F.
Published: February 2008

A short‐term, small‐scale ice motion model was developed in order to facilitate the comparison of various conceptual and numerical formulations. This ice motion model is comprised of two basic components: a momentum balance or equation of motion component and a thickness distribution component. The equation of motion component includes the standard terms relating to air stress, water stress, Coriolis force, sea surface tilt, and internal ice stress. In order to describe the internal ice stress, three different constitutive laws were implemented: a standard linear viscous constitutive law, a closed‐form viscous plastic constitutive law, and a piecewise linearized viscous plastic constitutive law. For the thickness distribution component, a choice of either a two‐level or a multicategory model may be used to characterize the spatial and temporal changes in the thickness distribution of the ice cover. The governing equations for both of these basic ice motion model components are presented. The finite element method based on the Galerkin method of weighted residuals is used to spatially integrate the resulting set of nonlinear partial differential equations while a standard recurrence relation is employed for the temporal integration. The selection of an Eulerian or Lagrangian description for both the equations of motion and the thickness distribution equations is also included in the model development. In order to investigate the response of the developed ice motion model and its various options, the model was applied to a 4‐day ice motion episode in the southern Beaufort Sea. The response of the ice motion model was quantified in terms of the magnitude and the spread of the difference between calculated and observed ice displacements. Comparison between the various model options is included and their effect on the overall model performance is discussed.
Reflected optical second‐harmonic generation in the study of naturally occurring organic films at the ocean surface
Authors: Asher W. E. , Frysinger G. S. , Korenowski G. M.
Published: February 2008

The nonlinear spectroscopic technique of second‐harmonic generation was used to study the surface properties of bulk and microlayer sea water samples. It was found that the majority of the surface active components present in the seawater were present in the microlayer sample. Increases in film pressure correlated with increases in the second‐harmonic generation signal for the microlayer samples studied. This indicates that the second‐harmonic generation technique may be used to noninvasively study and determine the surface properties of seawater.
Balances of heat and momentum at 33.5°N, 127°W in the upper ocean during the Mixed‐Layer Dynamics Experiment
Authors: Paduan Jeffrey D. , de Szoeke Roland A. , Richman James G.
Published: February 2008

The balances of momentum and heat were studied for an 18‐day period in October–November 1983 during the Mixed‐Layer Dynamics Experiment at a site 650 km off central California. Data were collected from a drifting platform from which surface winds, air temperature, and ocean currents (from a string of vector‐measuring current meters) were measured. The position of the unattended drifter was determined by LORAN‐C on the platform. Currents in the surface layers were partially wind driven and partially coherent with low‐frequency currents at deeper levels. The latter currents, though coherent, were sheared, turned significantly with depth, and were presumably nearly geostrophically balanced. Vertically differencing velocity in the mixed layer improved the balance between wind and current by removing a portion of the geostrophic flow. The complex correlation coefficient between wind stress and current integrated to 38 m was 0.76 for current relative to 38 m yet only 0.36 for the undifferenced case. Surface heat flux magnitudes were not clearly reflected in the heat content of the near‐surface layers although variability was. Over the first 13 days of the experiment, the correlation coefficient between daily‐averaged heat flux and heat content change was 0.89, but average net surface heating was 60 W m−2, while the average heat content change in the top 38 m was only 10 W m−2. Heat balance must have been restored by advection. Vertical advection was rejected as an explanation because heat content changes in layers defined by isotherm depths showed similar imbalances with regard to surface heating. Horizontal advection was estimated by two methods: (1) by using sparse thermistor chain tows around the current meter drifter to construct a history of horizontal temperature gradients and (2) by using current profiles at moderate depths to estimate pressure gradients (essentially through geostrophy), hence buoyancy gradients (by the hydrostatic relation), hence temperature gradients. The two methods gave very different estimates, though both tended to redress the imbalance between surface heat flux and local heat storage.
A computational method for estimating sea ice motion in sequential Seasat synthetic aperture radar imagery by matched filtering
Authors: Collins Michael J. , Emery William J.
Published: February 2008

There have been several recent studies outlining attempts to extract Arctic sea ice motion from Seasat synthetic aperture radar imagery (Hall and Rothrock, 1981; Curlander et al., 1985; Fily and Rothrock, 1986) These studies all highlighted the need to automate, or at least minimize, the amount of operator intervention involved in the procedure. This paper presents the results of a fully automated system to extract ice motion from an image pair. The problem of finding a field of displacements between two sequential images is posed as an estimation problem, and the system is shown to be an optimal solution to this problem. The performance of the system is shown to be superior to ice motion systems that use only cross correlation. The shortcomings of the system, and of area correlation in general, are outlined, and ways of overcoming them are suggested.
Fitting models to inadequate data by enforcing spatial and temporal smoothness
Authors: Thacker William Carlisle
Published: February 2008

When observations are too sparse to determine the state of a dynamical model, it is necessary to make use of prior knowledge or prejudice. The approach discussed here is to require that the model state be the best smooth fit to the sparse data. The requirement of smoothness is enforced by introducing bogus data, which correspond to hypothetical observations that properties such as slope, curvature, or temporal tendency of model fields have zero values within some specified accuracy. The bogus data serve to increase the effective ratio of data to model degrees of freedom. The concept of bogus data allows a bias toward smoothness to be incorporated easily into the adjoint method for fitting time‐dependent models to asynoptic data. Computational examples using a simple three‐wave model show that reasonable fits can be obtained even when the number of real data is considerably less than the number of model degrees of freedom.
A numerical study of mesoscale ocean eddy interaction with a marginal ice zone
Authors: Smith David C. , Bird Arlene A. , Budgell W. Paul
Published: February 2008

Observations made during the marginal ice zone experiment (MIZEX) program indicated the presence of mesoscale eddies in a marginal ice edge ocean front in the East Greenland Current. The eddies range in scale from 5 to 80 km. Physical mechanisms including barotropic and baroclinic instability may be responsible for their presence. Although the evolution of isolated eddies at low latitudes has been the focus of many studies, the effect of ice cover on eddy evolution and vice versa has had less attention. Here, an examination of the interaction of isolated mesoscale ocean eddies with a marginal ice zone is presented. Results are obtained with a two‐layer nonlinear primitive equation ocean model and a coupled free‐drift ice model. In the absence of winds, the ice equilibrates rapidly to the ocean eddy velocity, resulting in radial ice motion. The ocean sea surface slope is found to provide an important force in the ice momentum balance in no‐wind simulations. With along ice edge 10 m/s winds, the ice responds largely to the wind and to a lesser extent to the ocean eddy. Under ice the ocean eddy can be rapidly eroded by wind‐driven ice motion. Cyclones (anticyclones) are rapidly destroyed by along ice edge winds with ice on the left (right).
Variability of the East Australian Current over most of its depth and a comparison with other regions
Authors: Mulhearn P. J.
Published: February 2008

A knowledge of the structure of current systems over their total depth, especially of western boundary currents such as the East Australian Current, is important in assessing global transports. Many measurements have been performed on these currents and the generation of climatological statistics for them should assist in assessing the representativeness of conditions obtained in any particular study. To examine the structure, in a statistical sense, of the East Australian Current system over most of its depth, archived hydrological station data are analyzed to produce profiles of various statistics of temperature and dynamic height perturbations (e.g., means, standard deviations, and correlation coefficients as a function of vertical separation). These are compared with the same statistics derived for two other locations with less vigorous mesoscale perturbations. Nondimensionalized geostrophic velocity profiles are also obtained. The method uses all the individual profiles from a given area and does not need series of oceanographic sections or areal surveys. It is found that the magnitude of perturbations and their vertical length scales are larger in the East Australian Current region than in either of the other two locations.
Equatorial Kelvin waves embedded in mean flow, with application to the deep jets
Authors: Ponte Rui M.
Published: February 2008

Stationary equatorial Kelvin wave solutions are forced below the thermocline by a vertical velocity representing large‐scale convergence or divergence patterns associated with the upper ocean circulation, in the presence of a mean westward flow U mimicking the Equatorial Intermediate Current. For constant U, solutions exhibit a dominant vertical scale as given by McCreary and Lukas (1986), but in the presence of vertical mean shear, considerable changes in amplitude and vertical scale of the waves with depth occur. The combined effects of shear and linear friction on both the oscillatory and decay vertical scales of the wave flows are examined for typical shear profiles. In general, the faster the mean flow is at the top, the deeper energy is able to propagate before being dissipated. Shallow critical layers (where U = 0) may provide a barrier to the penetration of wave energy to great depths. The relevance of these ideas to the dynamics of the equatorial deep jets is discussed.
Analysis of ship‐generated surface waves using a method based upon the local Fourier transform
Authors: Wyatt Donald C. , Hall Robert E.
Published: February 2008

Laser wave slope meter data of the surface wake of the U.S. Navy tug, USS Quapaw, is analyzed using a method based upon the local Fourier transform (LFT). Three cases at ship speeds of 8.0, 7.82, and 7.85 m/s. are analyzed (JOWIP 8–2, 9–2, and 9–3). The results are compared with a simple kinematical surface wake model. The analysis reveals several physical phenomena, including the steady Kelvin wake, a region of unsteady ship‐generated waves, and the turbulent wake. The Kelvin wake includes the transverse waves, the diverging waves, and the cusp line, which appears at an angle of about 19.5° off the turbulent wake and serves as the outer boundary for the wake. Although most of the Kelvin waves are linear, there is an indication of nonlinearity, as evidenced by a bound second harmonic, just inside the cusp line. The unsteady waves appear within the region about ±7° off the turbulent wake and have wavelengths between a few centimeters and about 2 m. In the region of the short diverging Kelvin waves and the unsteady waves, the wave slope meter data sets in JOWIP 8–2, 9–2, and 9–3 are significantly different: JOWIP 9–2 data contains the largest amount of slope variance near locations predicted by Kelvin wave theory, JOWIP 9–3 contain the largest slope variance at locations predicted for unsteady waves, while JOWIP 8–2 contains low values of steady and unsteady waves. The differences could be due to intermittency in the sources for the waves or, in the case of JOWIP 8–2, to decay due to surfactants. The short diverging Kelvin waves and the unsteady waves can serve as Bragg waves for a range of radar wavelengths and orientations. At some locations in the JOWIP 9–2 and 9–3 data sets, the slope variance integrated over a range of L band Bragg waves attains peak values that are comparable to the same integrated slope variance in a wind wave spectrum at moderate wind speed. Also discussed is the general utility of the LFT method for the analysis of intermittent, broadband, oceanographic vector data.
On the time‐averaged flow of quasi‐geostrophic wind‐driven gyres
Authors: Marshall J. C. , Nurser A. J. G. , Brugge R.
Published: February 2008

The time‐averaged flow of a three‐layer eddy‐resolving quasi‐geostrophic ocean model is considered in the light of two analytical models of wind‐driven gyres, in which the vertical structure of the gyre is set by assuming that potential vorticity is uniform beneath layers exposed to forcing. The first, due to Young and Rhines (1982), supposes that the depth‐integrated meridional transport is set by the imposed wind‐stress curl. In contrast, in the baroclinic Fofonoff gyres of Marshall and Nurser (1986, 1988), inertial aspects of the flow are emphasized without imposing a Sverdrup constraint. The mean fields from the model are seen to lie between the two extremes represented by these analytical solutions. In the interior of the gyre away from inertial boundary currents and jets, the Sverdrup constraint is obeyed, and the mean flows resemble the Young and Rhines solution. However, the essential character of the overspun recirculation, including the weakly depth‐dependent nature of the currents in its return flow, seem to be well captured in the inertial limit considered by Marshall and Nurser.
Kinematics of CO2 fluxes in the tropical Atlantic Ocean during the 1983 northern summer
Authors: Garçon Véronique C. , Martinon Laurent , Andrié Chantal , Andrich Patrick , Minster Jean‐François
Published: February 2008

CO2 evasion within the Atlantic equatorial belt (5°N–5°S) increases from the east to the west [Andrié et al., 1986]. Many factors contribute the variations of pCO2 in the equatorial surface waters. To assess their relative importance, a kinematic box model is developed. A 2°×2° box whose depth is defined by the 24.90‰ isopycnal level flows westward from 4°W to 38°W within the Equator‐2°S band with the south equatorial current. Time (zonal) evolution of nitrate, total CO2, total alkalinity and mass, and of the corresponding water pCO2, are simulated taking into account advection, meridional divergence, diffusion, biological activity, and gas exchange. Initial and boundary conditions are taken from the FOCAL 4 (July‐August 1983) data set. The flow field is based on climatology. The ability of the model to reproduce the observed FOCAL data, as well as the correct orders of magnitude of the adjustable parameters KV (vertical mixing coefficient) and Jmax (maximum nitrate uptake rate), as compared with independent estimates, suggests that all major terms controling the equatorial Atlantic CO2 values are considered. Advection fluxes dominate the CO2 balance. The sinks due to biological consumption approximately balance the inflow from the core of the Equatorial Undercurrent by diapycnal mixing. Calculated new production JC values vary from east to west between 17 and 1 mol C/m2/yr, given Kv = 5 × 10−4 m2/s. Isopycnal mixing is negligible. Degassing plays a minor role in the total CO2 budget. Estimating the biological uptake of nutrients should be a very useful constraint for estimating equatorial divergence.
Estimating the thickness of sea ice
Authors: Bourke Robert H. , Paquette Robert G.
Published: February 2008

Sea ice freeboard, thickness, and snow depth were measured from a series of closely spaced (5 to 10 m) drill hole sites from five free‐floating multiyear ice floes in the Beaufort Sea during the spring of 1986 and 1987. A regression of ice thickness on ice draft was performed on the data from each floe and for the combined data set. Regressions were performed both on nonscreened data and data screened to eliminate pressure ridges. A predictive equation was developed permitting the total ice thickness to be estimated from measured values of ice draft, e.g., from submarine echo sounder traces: t = 1.115 d. The keel‐sail height ratio for pressure ridges was found constant at 3.3 to 1, in agreement with past observations.
Observational verification of a quasi real time simulation of the tropical Pacific Ocean
Authors: Hayes S. P. , McPhaden M. J. , Leetmaa Ants
Published: February 2008

Time series of upper ocean temperature and currents in the equatorial Pacific simulated by a numerical general circulation model run in nearly real time are compared to observations for the period August 1985 through May 1987. The model was forced by monthly mean wind stress and climatological air‐sea heat flux. Comparisons with observations near the equator in the western (165°E), central (140°W), and eastern (110°W) Pacific are discussed. Simulated sea surface temperature (SST) was too cool in the eastern Pacific and too warm in the western Pacific. Largest rms deviations were in the east and exceeded 2°C. On and north of the equator in the eastern Pacific, SST and thermocline depth fluctuations on seasonal and monthly time scales were prominent. South of the equator, seasonal variability dominated. The model simulations often reproduced the amplitude and phase of the seasonal changes but not the higher‐frequency variability. Model runs which included monthly assimilation of upper ocean temperature observations were included in the study. None of the comparison time series were incorporated in the assimilation. Inclusion of thermal observations generally improved agreement of simulated and observed time series. This improvement was largely due to reduction in the mean offsets of SST and thermocline depths. Data assimilation did little to improve the month‐to‐month differences in thermocline depth. In addition, south of the equator in the eastern Pacific, relatively large, systematic intra‐month SST deviations occurred. These deviations corresponded to an erroneous heat flux of about 80 W m−2 and indicated problems in the simulated upper ocean circulation. Although no velocity data were included in the assimilation, the improved model thermal structure led to improved velocity simulation at some locations. No comparisons indicated large‐amplitude spurious velocity varitions which could be associated with data assimilation transients.
Investigations on turbulent fluctuations of particle concentrations and relative humidity in the marine atmospheric surface layer
Authors: de Leeuw Gerrit
Published: February 2008

Results are presented from measurements on the variation of sea spray concentrations and relative humidity (RH) in the lower 15 m of the marine atmospheric surface layer. Particle total area concentrations were determined from measurements of near‐forward scattering intensities with an optical device. Both time dependent variations and mean values were recorded as a function of height from 0.5 to 15 m. The profiles of particle total area concentrations and relative humidity show trends similar to previously published particle concentration profiles measured with a rotorod impaction sampler. Hence our previous results are confirmed by using a completely different technique, based on different physical principles. The time dependent measurements show that fluctuations in the particle total area concentrations are correlated with the motions of the waves under the sampler. Spectral analyses reveal a height dependent frequency, which may be due to the wind profile, to the wave‐induced turbulence structure, and to changes in the particle size distribution. Simultaneous time‐resolved measurements of RH do not reveal a distinct wave influence. This analysis indicates that water vapor is transported by turbulent diffusion on a time scale much faster than 1 s. The time scale for the response of the largest sea spray particles to the surface displacements is estimated to be longer than 5 s. Theoretical implications are indicated. The results have implications for humidity transport and the constant flux assumption in the surface layer and may be applied to study interaction between wind and waves in the open ocean.
Acoustic tomography in the Florida Strait: Temperature, current, and vorticity measurements
Authors: Ko D. S. , DeFerrari H. A. , Malanotte‐Rizzoli P.
Published: February 2008

Various aspects of near‐bottom ocean variability, including the alongshore temperature and current, the cross‐slope temperature, the onshore‐offshore flow, and the area‐averaged relative vorticity on the western slope of the Florida Strait near 27°N have been measured with a three‐point acoustic reciprocal transmission. Three pairs of reciprocal travel time data were extracted from acoustic pulse responses of a 1983 (August) triangular transmission experiment. Temperatures and currents were estimated from the sum and difference, respectively, of reciprocal travel times, and an area‐averaged vorticity was estimated from circulation following Stokes' theorem. The estimations agree well with the concurrent direct measurements at two nearby moorings. Low‐frequency fluctuations of temperatures, currents, and vorticity show a strong weekly period, clearly and consistently describing a near‐bottom onshore‐offshore water movement on the western slope in association with the Gulf Stream meanders.
Wind stress and heat fluxes observed during winter and spring 1986
Authors: Blanton Jackson O. , Amft Julie A. , Lee Dana K. , Riordan A.
Published: February 2008

Data collected during winter from a set of offshore buoys and coastal meteorological stations on the continental shelf of the southeastern United States were analyzed as part of a meteorological and oceanographic study of the genesis of winter cyclones (Project GALE). The analyses were designed to describe the temporal and spatial variability of wind stress and heat flux over the shelf. While there were some episodes of strong spatial variations in wind stress, the wind field, on the whole, was remarkably uniform. Largest gradients were associated with times immediately preceding outbreaks of cold continental air over the shelf. Maximum heat fluxes occurred during these cold air outbreaks and reached as high as 1400 W/m2 on the outer shelf near the Gulf Stream. The largest wind stress values occurred in the cross‐shelf component and reached 0.7 Pa. The cross‐shelf wind was highly correlated with the geostrophic wind, but there were significant ageostrophic deviations in the alongshelf component. These deviations were correlated with fluctuations in sea‐air temperature differences. The fluctuations in heat flux are thought to modulate the alongshelf component by inducing added vorticity in the planetary boundary layer over the scale width of the continental shelf.
Stability of a coastal upwelling front: 2. Model results and comparison with observations
Authors: Barth J. A.
Published: February 2008

A two‐layer shallow water equation model is used to investigate the linear stability of a coastal upwelling front. The model features a surface front parallel to a coastal boundary and bottom topography which is an arbitrary function of the cross‐shelf coordinate. The existence of unstable waves on the model coastal upwelling front, as suggested by the general stability theorem developed in a companion paper, is confirmed by solving directly the linearized equations of motion. The unstable wave motions are frontally trapped and dominant in the upper layer. The wave propagates phase in the direction of the basic state flow, and the primary energy conversion is via baroclinic instability. The effect of varying the model parameters is presented. Moving the front closer than ∼2 Rossby radii to the coastal boundary results in a decrease in the growth rate of the fastest growing wave. Increasing the overall vertical shear of the basic state flow, by either decreasing the lower layer depth or increasing the steepness of the interface, results in an increase in the growth of the fastest growing wave. A bottom sloping in the same sense as the interface results in a decrease of the growth rates and alongfront wave numbers of the unstable waves in the system. Linearized bottom friction is included in the stability model and results in a decrease in the growth rates of the unstable waves by extracting energy from the system. Since the unstable mode is strongest in the upper layer, bottom friction will not stabilize the upwelling front. A comparison between the predictions from the simple two‐layer model and observed alongfront variability for three areas of active upwelling is presented. Reasonable agreement is found, suggesting that the observed alongfront variability can be interpreted in terms of the instability of a coastal upwelling front.
On the ability of rough surface scattering approximations to predict hydrodynamic modulation of the ocean radar cross section: A numerical study
Authors: Durden Stephen L. , Vesecky John F.
Published: February 2008

Microwave backscatter from the ocean surface away from normal incidence is often described by either the Bragg or the two‐scale (composite) scattering model. In this report we study the ability of the Bragg and two‐scale models to predict spatial or temporal modulation of the normalized radar cross section (σ°) caused by spatial or temporal changes in ocean surface roughness, such as those due to the presence of internal wave surface effects. Our investigation is accomplished by comparing Bragg and two‐scale cross section models with cross sections calculated by a numerical electromagnetic approach. We take the numerical calculations as our standard of comparison because they are not limited by the approximations of the Bragg and two‐scale models. We find that for slightly rough surfaces, the Bragg model is very accurate. However, for much rougher surfaces, the two‐scale model is superior and should be used for calculating both the absolute cross section and the change in cross section due to spatial or temporal modulation of the ocean wave height spectrum. The conclusions reached are relevant to interpretation of synthetic aperture radar (SAR) images of ocean internal waves collected in the SAR Internal Wave Signature Experiment (SARSEX) as well as other SAR remote sensing problems involving enhancement of ripple wavelength waves by ocean surface straining.
A year of Bowen ratios over the frozen Beaufort Sea
Authors: Andreas Edgar L.
Published: February 2008

The data set obtained during the Arctic Ice Dynamics Joint Experiment (AIDJEX) contains almost a year of 6‐hourly measurements of sensible (Hs) and latent (HL) heat fluxes over multiyear ice in the Beaufort Sea. Here I compute the Bowen ratio, Bo = Hs/HL, for each set of observations and plot histograms of Bo values for the four seasons, spring, summer, autumn, and winter. In every season except summer the Bowen ratio is as likely to be negative as positive; the histograms are, thus, generally bimodal. The Bowen ratio shows a seasonal progression that is related to the seasonally averaged air temperature. In summer, when moisture exchange is most rapid, the peaks in the histogram are near Bowen ratios of 1 and −1. In winter, when the rate of moisture exchange is smallest, the peaks are at Bo>20 and Bo < −20. Spring and autumn are transition seasons with a broader range of values.
Forcing the volume transport through Drake Passage
Authors: Johnson Mark A.
Published: February 2008

Time series of the zero wind stress curl latitude and of the zonally averaged wind stress curl magnitude are used to drive volume transport in a linear, one‐layer, quasi‐geostrophic numerical model with a Drake Passagelike gap. The wind stress curl time series, computed from observed winds and low‐pass filtered to retain only seasonal time scales, are used to test the hypothesis that changes in the latitude of zero wind stress curl influence volume transport fluctuations through Drake Passage. Model results show that variations in the latitude of zero wind stress curl are coherent and in phase with model volume transport at periods greater than 30 days. When the time series of the latitude of zero wind stress curl are analyzed with transport accelerations both from the model and from Drake Passage observations, coherence squared between acceleration and the integrated wind stress curl are good. Both model experiments and Drake Passage field measurements suggest that variations in the latitude of zero wind stress curl are an important mechanism in modifying volume transport fluctuations in the Antarctic Circumpolar Current.
Modeling new production in upwelling centers: A case study of modeling new production from remotely sensed temperature and color
Authors: Dugdale Richard C. , Morel André , Bricaud Annick , Wilkerson Frances P.
Published: February 2008

A method has been developed for estimating new production in upwelling systems from remotely sensed surface temperatures. A shift‐up model predicts the rate of adaptation of nitrate uptake. The time base for the production cycle is obtained from a knowledge of surface heating rates and differences in temperature between the point of upwelling and each pixel. Nitrate concentrations are obtained from temperature‐nitrate regression equations. Surface distributions of new production as nitrate uptake are produced. The model was developed for the northwest Africa upwelling region, where shipboard measurements of new production were available. It can be employed in two modes, the first using only surface temperatures, and the second in which CZCS color data are incorporated. The color data provide an estimate of chlorophyll which is used to estimate the diffuse attenuation coefficient and the particulate nitrogen concentration pixel by pixel. Vertical integrations for each pixel can be made when the euphotic zone is well mixed and homogeneous for phytoplankton. Future applications of the model should be on a region‐by‐region basis where shipboard measurements are available to calibrate the algorithms and validate the results. The major advance offered by this method is the capability to estimate new production on spatial and time scales inaccessible with shipboard approaches.
The variability of atmospheric equivalent temperature for radar altimeter range correction
Authors: Liu W. Timothy , Mock Donald
Published: February 2008

Two sets of data were used to test the validity of the presently used approximation for radar altimeter range correction due to atmospheric water vapor. The approximation includes an assumption of constant atmospheric equivalent temperature. The first data set includes monthly, three‐dimensional, gridded temperature and humidity fields over global oceans for a 10‐year period, and the second comprises of daily or semidaily rawinsonde data at 17 island stations for a 7‐year period. It is found that the standard method underestimates the variability of the equivalent temperature, and the approximation could introduce errors of 2 cm for monthly means. The equivalent temperature is found to have a strong meridional gradient, and the highest temporal variabilities are found over western boundary currents. The study affirms that the atmospheric water vapor is a good predictor for both the equivalent temperature and the range correction. A relation is proposed to reduce the error.
Recovery of the sea level signal in the western tropical Pacific from Geosat altimetry
Authors: Cheney Robert E. , Miller Laury
Published: February 2008

In a previous paper we pointed out deficiencies in our Geosat results for the western tropical Pacific (130°E to 160°E), where less than half of the sea level signal amplitude was recovered. The suspected cause was the quadratic orbit error model, which, together with the relatively short arcs used, resulted in severe signal damping. (Over most of the Pacific, where long arcs were available, this process removed an average of only 10% of the ocean signal.) We have since reprocessed the first 2.5 years of Geosat data for the western region using longer arcs extending into the southern ocean together with a linear orbit error model. Dramatic improvement in the sea level time series was obtained over most of the area, with approximately 90% of the sea level variability recovered, although continued signal damping was still noted in the extreme west. These results confirm our hypothesis with regard to orbit error and at the same time suggest that other error sources such as water vapor were much less significant. These new sea level records provide an improved description of the western Pacific prior to and during the 1986–1987 El Nino.
Statistical relationships between local sea level and weather with Florida‐Bahamas cable and Pegasus measurements of Florida Current volume transport
Authors: Maul George A. , Mayer Dennis A. , Bushnell Mark
Published: February 2008

Straits of Florida sea level is studied as a measure of Florida Current volume transport because sea level provides an independent time series both (1) for detecting changes in the calibration of the Florida‐Bahamas submarine cable which has been operating since 1982, and (2) as a measure of the strength of the Gulf Stream System dating to the 1930s when cable measurements were not available. Accordingly, tide gauge records from Haulover Beach (Miami) and Lake Worth, Florida, and from Cat Cay and Settlement Point, The Bahamas, together with Miami weather and cable voltage, are correlated with each other and with discrete volume transport estimated from all Pegasus data taken during the intensive 1982–1984 Subtropical Atlantic Climate Studies (STACS) observations. Time domain linear correlation coefficients between these 132 Pegasus values and cable voltage observations, Cat Cay minus Haulover Beach sea level, and Haulover Beach sea level only, are 0.91, 0.55, and −0.76, respectively, but for the 1982–1988 time series, Cat Cay minus Haulover Beach is better correlated with cable observations (0.62) than is Haulover Beach alone (−0.44). Frequency domain modeling computations reveal that the sea level and cable data are organized in such a way that most of their energy lies below frequencies of 30−1 cycles per day (cpd). In a subseasonal band (up to and including the semiannual frequency) 49% of the cable energy and 59% of the Haulover Beach energy can be accounted for using 30‐day low‐passed data. A subseasonal frequency response function has been tentatively identified that relates both Florida sea level and Bahamas‐Florida sea level difference to cable observations and can successfully account for more than 60% of the observed cable variance (standard error is ±1×106 m3 s−1). Further, negative Florida sea level only and Bahamas‐Florida sea level difference have different phase relationships with cable observations; it is suggested that steric effects could be responsible for the phase shifts.
Corrected spectra of wind speed and significant wave height
Authors: Monaldo Frank
Published: February 2008

In a recent paper (Monaldo, 1988) I presented spatial spectra of wind speed and significant wave height (SWH) variance. The spectra were calculated from spatial variations of wind speed and SWH as measured by the Seasat and Geosat radar altimeters. It has recently come to my attention that these spectra were incorrectly calculated. The shape of the corrected wind speed variance spectra no longer agree with theoretical predictions by Thompson (1973). The expected differences between two altimeter wind speed or SWH measurements separated a given distance apart were calculated in the time domain and not by inverse Fourier transformation of the spectra. The expected differences given by Monaldo (1988) are therefore unaffected by the errors in the calculation of the SWH and wind speed spectra.
Statistics of mixing patches observed in the Sargasso Sea
Authors: Rosenblum L. J. , Marmorino G. O.
Published: February 2008

Temperature measurements made with a towed thermistor chain in the seasonal thermocline of the Sargasso Sea have been converted to sections of fine‐structure “activity” by normalizing values of meter‐scale horizontal fluctuations by the local vertical gradient. Individual “patches” of high activity are then separated from background and examined for length L, height H, aspect ratio L/H, and signal intensity. Over the range 0.5m<H<20m and 30 m < L < 2 km, the aspect ratio tends to be about 100. Similar to reports of turbulent patches measured with velocity microstructure profilers, numbers of patches decrease with increasing patch height and length, and large patches tend to have higher activity levels.
Aspects of the Kelvin wave response to episodic wind forcing
Authors: Giese Benjamin S. , Harrison D. E.
Published: February 2008

Episodes of westerly wind are an important aspect of surface stress variability in the western Pacific. During El Niño‐Southern Oscillation periods, the presence of such wind episodes comprises much of the low‐frequency relaxation of the trades over the central and western Pacific. In this paper we describe the oceanic Kelvin pulse response to a single idealized episode of westerly wind stress, using results from linear theory as well as from a 27‐level general circulation model. Linear theory predicts that an episode of westerly wind will excite a train of equatorially trapped Kelvin pulses. The amplitude and longitudinal structure of the forced ocean Kelvin pulses varies as a function of baroclinic mode and the wind patch properties. Because of changing vertical thermal structure across the Pacific, the vertical structure of Kelvin pulses is altered as they propagate away from the forcing region. When stratification typical of the western and eastern Pacific is used, the conservation of energy flux predicts a reduction of surface currents associated with the first baroclinic mode and an enhancement of surface currents associated with the second baroclinic mode. The idealized wind anomaly is also used to drive an ocean general circulation model. When the wind anomaly is weak the model Kelvin response agrees with predictions of linear theory. For more realistic strong forcing there are three important deviations from linear theory: the amplitude of low baroclinic modes increases; the amplitude of higher baroclinic modes decreases; and the phase speed increases. In the presence of realistic oceanic background conditions, response in the equatorial waveguide is complicated by the equatorial undercurrent, a sloping thermocline and instability waves. Model sea surface temperature warming at the coast of South America is dominated by the second baroclinic mode, consistent with the results derived from linear theory.