bluuebunny/crossref_metadata_2025 huggingface dataset:
Bubbles in the near‐surface ocean: A general description
Authors: Wu Jin
Published: February 2008

Combining recently reported measurements with those reviewed earlier, a general description of concentrations and size distributions of bubbles at various depths in the near‐surface ocean under a wide range of wind velocities is proposed.
Aluminium as a tracer of the deep outflow from the Mediterranean
Authors: Measures C. I. , Edmond J. M.
Published: February 2008

Dissolved aluminium has been determined in sea water samples from the southwestern Alboran Sea and the Gulf of Cadiz. Partial dissolution of eolian dust is responsible for the elevated levels of dissolved Al that were found in both the Western Mediterranean Deep Water (WMDW) (111.5 nM) and Levantine Intermediate Water (LIW) (86.2 nM) masses in the Alboran Sea. The Al concentration in the outflowing plume of Mediterranean water sampled in the Gulf of Cadiz is also elevated (72 nM). These enhanced signals and the differences between them are used in a three water mass system to calculate a maximum 10% contribution of Western Mediterranean Deep Water to the outflow plume, considerably less than has been previously supposed. While the synoptic nature of this study and the possibility of involvement of other water masses in the outflow plume leave scope to increase the participation of WMDW in the outflow plume, these measurements indicate the potential of a nontransient tracer in elucidating the component parts of complex flow systems.
Entrainment and homogenization of a passive tracer in a numerical model gyre
Authors: Pickart Robert S.
Published: February 2008

The mechanism by which tracer is entrained into a spatially decaying gyre from an external source is examined, as well as the homogenization which subsequently occurs. A simple advective‐diffusive numerical model is used whose streamlines consist of an elongated gyre situated beside a distinct boundary current, which inputs tracer into the domain. This is meant to represent the deep cyclonic recirculation of the Gulf Stream and adjacent deep western boundary current. A shear flow analysis shows that two parameters dictate the manner in which tracer penetrates across streamlines of the gyre: a peclet number and a parameter which measures the strength of the cross‐stream shear. The large values of these parameters cause a plume of tracer to spiral inward toward the center of the gyre. At steady state the tracer which has accumulated in the gyre becomes homogenized. The size of this uniform area is related to the extent to which the spiral penetrated the gyre and decreases with increasing diffusivity, as several examples illustrate.
Transient tracers as a problem in control theory
Authors: Wunsch Carl
Published: February 2008

The problem of interpreting transient tracer surveys in the ocean is formally identified as corresponding to placing a “terminal constraint” on a “distributed system boundary control problem.” The mathematics available in control theory can then be brought to bear on the tracer data. Some of control theory is reviewed in the context of a simple tracer example to isolate the major issues. To use a transient tracer to invert for flow and mixing rates involves a two‐step process: start with an initial model, found independently, and determine if acceptable boundary conditions drive the model to reproduce the interior transient tracer at the observation times. If the model succeeds in that reproduction, one stops; the model is adequate and need not be changed. Only if this test fails does one obtain constraints on the fluid flow and mixing, which can be invoked in parameter estimation techniques of control theory. Terminal constraint observations can also be used to estimate the tracer concentrations at earlier times using a smoothing filter.
Bubbles in the close vicinity of breaking waves: Statistical characteristics of the generation and dispersion mechanism
Authors: Baldy Serge
Published: February 2008

A direct investigation focussed on the vicinity of the interface of bubble populations generated by different breaking wave fields in a wind‐water simulation facility is presented. Bubble size spectrum over a diameter range 60 μ ≤ d < 3000 μ, obtained from a laser‐based single‐particle technique is given for different depths in the vicinity of the interface and different conditions of wave age and wind. Self‐consistent results are obtained if the dimensionless depth z* = z/H⅓, where H⅓ is the significant wave height, is used and two partly overlapping zones are apparent: (1) next to the surface the spectral shape is found to be invariant, and the global bubble concentration decreases rapidly with depth and (2) at lower levels the concentration of small bubbles is roughly constant, and the slope of the size spectrum increases with depth. Distributions of time intervals between bubble arrivals in the sampling volume and bubble speed estimations obtained from bubble time of residence in this volume further suggest bubble clusters in the upper zone that fade out in the lower zone. In addition, bubble concentration is found to increase very rapidly at a given depth with wind speed and more rapidly for large bubbles than for small ones. Bubble cluster, speed, and size distribution variations with depth furnish evidence for the identification of the upper zone with a generation zone and the lower with a dispersion zone.
Field intercomparisons of electromagnetic current meters
Authors: Guza R. T. , Clifton M. C. , Rezvani F.
Published: February 2008

In order to assess the performance of current meters within and near the surf zone, data from biaxial electromagnetic current meters with spherical and open frame probe geometries were intercompared. One bottom‐mounted flow meter of each type was deployed in a mean depth of 7.0 m for 17 days, and two sensors of each type were deployed in a mean depth of 2.0 m for 5 days. Sensors in the shallow deployment were frequently in the surf zone. Hourly averaged mean flows measured by different sensor types are highly correlated, averaging above 0.98. The largest difference between measured mean flows is a constant bias, typically about 3.0 cm/s, which is roughly equal to the estimated accuracy of the sensor offset calibrations. Root‐mean‐square deviations from this constant bias are less than 2.0 cm/s, and are contributed to by errors in both gain calibration and sensor orientation. Comparisons of measured (surface gravity wave) oscillatory currents were made both between current meter types and with velocities inferred from the application of linear theory to pressure sensor data. Correlations between time series of UTrms (the rms total oscillatory velocity for a 1‐hour record) were all above 0.99 in 7.0‐m depth and averaged 0.95 for the shallow deployment. The average UTrms ratio (over all hour‐long records) was within 1.0 ±0.07 for all current meter pairs in both deployments, which is consistent with the estimated 5% uncertainties in the flow meter gain calibration. Typical fluctuations of the UTrms ratio of any spherical and open frame sensor pair about its mean ratio, indicative of flow meter gain distortions probably associated with variations in the hydrodynamic environment, were less than 0.04 for any one deployment. Ratios of UTrms from both deployments taken together suggest that the open frame sensor overresponds, relative to the spherical probe, by about 5% at low (about 10.0 cm/s) total (mean + UTrms) speeds, and underresponds by about 5% at higher total speeds of about 75 cm/s. Relative to pressure data and linear theory, both flow meter types overrespond at low total speeds and underrespond at high total speeds. We are, however, unable to determine whether these apparent gain deviations (of less than 10%) of the flow meters relative to pressure are associated with errors in the linear theory used to convert pressure to velocity, or with the response characteristics of the flow meters. Cross spectra between all sensors (including pressure) show high coherence and phase differences of a few degrees, and they suggest that the response of both flow meters is only slightly frequency dependent. Various practical difficulties in accurately measuring the flow‐induced microvolt potentials in electromagnetically noisy environments, with potentially interfering current meters, are discussed.
Microwave scattering from internal wave modulated surface waves: A shipboard real aperture coherent radar study in the Georgia Strait Experiment
Authors: Kwoh D. S. W. , Lake B. M. , Rungaldier H.
Published: February 2008

A shipboard continuous wave coherent dual‐polarized focused radar operating at 9.23 GHz, an optical device which detects the occurrence of specular reflection at the radar incidence angle, and a laser wave slope gauge have been used to investigate mechanisms governing radar backscatter from surface water waves as part of the Joint Canada‐U.S. Ocean Wave Investigation Project ( also known as the Georgia Strait Experiment ). The relative contributions of Bragg and specular backscatter mechanisms have been identified using the output of the optical specular detector and the radar backscatter polarization ratios. Analysis of the radar data together with data from the laser wave slope gauge ( Hughes and Dawson, this issue ) has been used to further identify the mechanisms governing radar backscatter and modulations of radar backscatter from surface water waves. It is found that specular reflection is very significant at 20°incidence angle and not negligible at 40°. It is found that when specular contributions are absent, the relationship between the surface wave slope and the radar backscatter is well described by Bragg theory. Specular contributions, when they occur, appear as large spikes on top of the Bragg return. Analysis of the wave slope data shows that the wave slope modulations at “ X band ” and “L band ” Bragg wave frequencies are comparable, in contradiction to simple Bragg wave modulation theories which predict that “ X band ” waves should exhibit much weaker modulations. Modulations of the radar backscatter in low seas are due primarily to Bragg scattering. As the wind speed increases, the hydrodynamic modulations tend to decrease, but the radar modulations can be enhanced by specular reflection. Possible additional contributions due to wedge diffraction type scattering at higher wind speeds may also be important but have not been addressed in this experiment. Exceptionally low radar returns were observed in some cases and may be due to the presence of organic films on the surface.
A comparison of measured surface wave spectral modulations with predictions from a wave‐current interaction model
Authors: Thompson D. R. , Gotwols B. L. , Sterner R. E.
Published: February 2008

Predictions from a wave‐current interaction model based on a wave action balance equation are compared with measured surface wave modulations induced by internal waves. The comparison involves relative modulations of the surface wave spectrum at wavelengths ranging from 0.2 to 1.0 m for wind speeds of 3.5 and 7 m/s. Good agreement is found between measurements and predictions for interactions with eight internal waves in two wave packets encountered during the Synthetic Aperture Radar Internal Wave Signature Experiment.
A comparison of empirical and dynamical hindcasts of low‐frequency, wind‐driven motions over a continental shelf
Authors: Chapman David C. , Lentz Steven J. , Brink Kenneth H.
Published: February 2008

Three approaches to hindcasting low‐frequency wind‐driven shelf currents, bottom pressure, and adjusted sea level are compared at two locations along the west coast of the United States. The “purely empirical” approach is a multiple input linear regression in which alongshelf wind stress records at various time lags are fit to the observations. The “empirical FOWE” (first‐order wave equation) approach assumes the dynamics of a single wind‐forced, frictionally damped, coastal‐trapped wave and adjusts the choice of free‐wave phase speed and frictional decay time to produce the best hindcast of the oceanographic observable. The “dynamical FOWE” approach uses the full theory of wind‐forced, long coastal‐1 rapped waves to hindcast observables with no adjustment of the free‐wave parameters to improve the fit. The results are similar for both hindcast locations. The purely empirical approach produces the best fit (in a least squares sense) but offers little dynamical insight. At both locations using more than two wind records (one local and one ∼500 km to the south) does not significantly improve the hindcast. The other two approaches perform nearly as well as the purely empirical approach. However, the empirical FOWE approach can lead to dynamical inconsistencies because the optimal phase speed may be different for different variables or for the same variable observed at different cross‐shelf locations. Further, local forcing along with remote forcing can bias the results, leading to lower phase speed and friction time estimates than are appropriate for the remotely forced signal. The dynamical FOWE approach offers the most physical insight and produces results which are consistent between variables and at different cross‐shelf locations. Further, it is the only approach of the three which can be used as a true forecasting tool. Its performance is, however, also degraded by strong local forcing, suggesting that more dynamical sophistication is required.
Multifocus processing of L band synthetic aperture radar images of ocean waves obtained during the Tower Ocean Wave and Radar Dependence Experiment
Authors: Tajirian E. K.
Published: February 2008

As part of the Tower Ocean Wave and Radar Dependence Experiment (TOWARD) objectives, the mechanisms of SAR imaging of ocean waves are investigated using L band SAR data over the Naval Ocean Systems Center tower. This paper provides experimental evidence needed to validate the differing hypotheses. Various processing methods are investigated to generate spectra with large degrees of freedom. The results show that waves traveling in the aircraft direction are most detectable at focus settings in the range 10.0–15.0 m/s, which is consistent with the Marine Remote Sensing Experiment (MARSEN) observations reported by Jain and Shemdin (1983). Waves traveling in the direction opposite to the aircraft are most detectable at settings equal to −5.0 to −15.0 m/s. The SAR imaging system acts as a low‐pass filter with the peak of the ocean wave height spectrum occurring at higher wave numbers compared with the peak in the SAR image spectrum.
On the transport of the Antarctic Circumpolar Current through Drake Passage and its relation to wind
Authors: Peterson Ray G.
Published: February 2008

A time series of net transport during 1979 of the Antarctic Circumpolar Current (ACC) through the upper 2500 m of Drake Passage was obtained as part of the International Southern Ocean Studies program by Whit worth (1983), and was presented in a revised form by Whitworth and Peterson (1985). Spectral analyses are performed on records comprising the revised series to determine the major sources of transport variability. Multiyear records from bottom pressure gauges at each side of the passage are then compared with fields of wind stress over the Southern Ocean. The net transport series is dominated in the subseasonal time scales by the lunar fortnightly and monthly tides and by baroclinic activity in the northern passage, processes that are independent of wind. For seasonal time scales, transport variability appears to be mainly barotropic, thus being well represented by bottom pressure records from each side of the passage. Comparisons of multiyear bottom pressure records with time series of the zonally averaged curl of wind stress in narrow bands along the perimeters of the ACC suggest that seasonal fluctuations in the strengths of the subtropical and subpolar atmospheric pressure systems flanking the ACC induce changes in the slope of the sea surface across the current. The dominant annual cycle in curl associated with the subtropical highs appears to influence sea level across the entire ACC, whereas the effects of the strong semiannual cycle in curl associated with the subpolar lows are confined to the southern portions of the current.
Evaluation of preliminary experiments assimilating Seasat significant wave heights into a spectral wave model
Authors: Esteva Dinorah C.
Published: February 2008

Preliminary experiments dealing with the assimilation of significant wave heights (SWH) from the Seasat altimeter into the National Oceanic and Atmospheric Administration Ocean Wave (NOW) global spectral model are discussed. For all experiments, National Meteorological Center‐analyzed winds for the Seasat period were used to drive the ocean surface. A hindcast was first made running the model with no assimilation for use as a control. The model was then run, using both simple replacement and a blending procedure to spread the influence of an observation to nearby grid points and for different assimilation frequencies. Forecasts of SWH from each assimilation run and the hindcasts are compared to the corresponding Seasat SWH estimate from subsequent satellite passes. Best results are obtained by using a 3‐hour assimilation frequency with simple replacement and by scaling the forecasted spectrum with the observed SWH. A mean improvement of 7.7% (±8.4% at the 90% confidence level) is thus achieved for the 10 days of the experiment, with a maximum improvement of 25%.
Coefficients for sea surface wind stress, heat flux, and wind profiles as a function of wind speed and temperature
Authors: Smith Stuart D.
Published: February 2008

Surface layer coefficients for wind profiles, wind stress, and heat flux in typical open sea conditions are briefly reviewed. Businger‐Dyer flux‐gradient relationships and a Charnock wind stress formula fit the empirical data and are dimensionally consistent. These have been solved by an iterative method, and the results are presented in a tabular form suitable for climatological calculations from marine wind and temperature data.
Mathematical characteristics of sea ice dynamics models
Authors: Pritchard Robert S.
Published: February 2008

Mathematical characteristics of plasticity models that describe large‐scale sea ice behavior are studied in an attempt to understand the models better and to compare the analytical results to observed ice behavior. Separate stress and velocity characteristics occur when a nonassociated flow rule is used. The two sets coincide for associated flow rules. The relative orientation of characteristic directions at a point can change depending on the stress state and the direction of plastic stretching. Two real velocity characteristic directions exist when the magnitude of shearing exceeds the magnitude of dilating, one exists when they are equal, and none exist when the shearing magnitude is less than the dilating magnitude. The system is hyperbolic, parabolic, or elliptic, respectively, in these conditions. Quasi‐steady elastic‐plastic and viscous‐plastic models have identical characteristics so that we cannot use the characteristics to discriminate between these models. Attention has been focused on the velocity characteristics with the desire to relate these directions directly to leads and other features observed in satellite imagery. The velocity characteristic curves cannot stretch, so that floes lying along a characteristic line neither approach nor separate from each other. Therefore the amount of open water will not change between these floes, which makes it unlikely that we will be able to observe fields of characteristic directions directly in satellite imagery. We have, however, observed velocity discontinuities across large‐scale lead systems, which are special characteristic curves. This result offers a way to use remotely sensed imagery directly to estimate part of the deformation field.
Satellite determination of the carbon dioxide exchange coefficient at the ocean‐atmosphere interface: A first step
Authors: Etcheto J. , Merlivat L.
Published: February 2008

Using 3 months of wind measurements made by the Seasat scatterometer and the relationship between the wind speed and the transfer velocity measured by Liss and Merlivat [1986], we deduce the exchange coefficient between air and sea for carbon dioxide on a worldwide scale. Global maps of the exchange coefficient, with a 10°×10° resolution, are made, neglecting the temperature dependence due to the Schmidt number and solubility temperature variations, as we show that the variations of these two parameters nearly compensate in the range of variation of sea surface temperature in the global ocean. The influence of the ice‐covered area of the ocean, where no gas exchange occurs, is taken into account. A mean value of the global average exchange coefficient is derived, equal to 3.63×10−2 mol m−2 yr−1 μatm−1. Notwithstanding that this number is based on a 3‐month period of wind measurements, it is consistent with previous estimates, such as the determination made from radioactive tracers (i.e., carbon 14, radon), the values computed using climatological charts, and estimates derived from large‐scale experimental wind data. The influence of our method of averaging on the results is studied using an example in the Gulf of Guinea. Using in situ wind and carbon dioxide measurements, we must stress the importance of seasonal variations in the determination of the net flux on an annual basis. Our results, combined with the global map of the partial pressure gradient of carbon dioxide established by Broecker et al. [1986], give an estimation of the net flux between air and sea of 1.17 1015g C yr−1, lower than usually expected (about 2 to 2.5 1015g C yr−1) from the global budget of the carbon dioxide cycle [Siegenthaler, 1983]. Considering the limitations of the data (only 3 months of wind speed measurements combined with low spatial resolution annual mean ΔPCO2 maps), this discrepancy is not surprising, especially as the result is very dependent on the ΔPCO2 used. Moreover, we study the consequences of assuming a spatially constant exchange coefficient for estimating a net flux. Such an assumption can result in a very large error.
Relationships between near‐surface plankton concentrations, hydrography, and satellite‐measured sea surface temperature
Authors: Thomas A. C. , Emery W. J.
Published: February 2008

In situ measurements of surface chlorophyll and zooplankton concentration are compared with in situ hydrographic measurements and infrared satellite images of the west coast of British Columbia for early winter and midsummer study periods. Maximum winter concentrations of chlorophyll (1.0 mg m−3) and zooplankton (2900 counts m−3) were found in colder, more stratified nearshore water. Warmer water over the middle and outer shelf consistently had the lowest chlorophyll and zooplankton concentrations (<0.2 mg m−3 and <1700 counts m−3, respectively). Correlations between winter loge transformed Zooplankton concentrations and surface temperature demonstrated that infrared satellite imagery explained 49% of the sampled zooplankton concentration variance. The winter association of specific chlorophyll concentrations with identifyable hydrographic regimes enabled the satellite imagery, in conjunction with an image‐derived salinity model, to explain 55% of the sampled chlorophyll variance. Maximum summer chlorophyll concentrations (>20.0 mg m−3) coincided with intermediate temperatures around the edge of an upwelling frontal zone with lower concentrations (≈5.0 mg m−3) in the coldest, most recently upwelled water. Lowest concentrations (<1.0 mg m−3) were present in vertically stratified regions of the shelf. A least squares fit nonlinear equation showed that satellite‐measured surface temperature patterns explained 72% of the loge transformed chlorophyll variance. In contrast with the above relationships, summer zooplankton concentrations were not consistently related to satellite temperature patterns. While peaks showed a qualitative association with higher chlorophyll concentrations at the outer edge of the upwelling area, surface temperature was a poor predictor of zooplankton concentration over the study area as a whole.
In situ measurements of marine particle settling velocity and size distributions using the remote optical settling tube
Authors: Spinrad Richard W. , Bartz Robert , Kitchen James C.
Published: February 2008

A remote optical settling tube was deployed in the deep ocean to obtain the first ever in situ distributions of particle settling velocity. These distributions were extrapolated to particle size distributions assuming several possible fixed and variable (with size) particle densities. Cumulative slopes of the particle size distribution (with fixed particle densities of 1.1–2.0 g cm−3) were consistent with earlier measurements made shipboard in the same region. Cumulative slopes obtained using a more realistic variable density (as would be the case for populations including zero‐order and first‐order aggregates) were considerably lower, suggesting that the remote settling tube is detecting aggregated material more accurately than can be done with impedance‐based particle size analyzers. This is supported by the fact that the particulate relative real refractive index was also computed from the specific beam attenuation coefficient and yielded expected results only when the variable density size distribution was applied. These data and analyses demonstrate that the remote optical settling tube can be used effectively (even at low concentrations of suspended material) for in situ determinations of the particle size distribution, especially where relatively fragile aggregates may be present.
Hindcasts and data assimilation studies with the WAM model during the Seasat period
Authors: Janssen Peter A. E. M. , Lionello Piero , Reistad Magnar , Hollingsworth Anthony
Published: February 2008

In the next decade a wealth of ocean surface data will become available through the launch of satellites such as ERS 1. We discuss the problem of how to make optimal use of this data. We have investigated the benefits of having a coupled wind‐wave analysis over the oceans for quality assessment of satellite data, for monitoring the performance of the algorithms (e.g., for the scatterometer), and for producing consistent wind and wave analyses. Using a reliable wave prediction scheme, the WAM model, we show how to cross validate altimeter wave height with scatterometer data and how to construct an analyzed wave model spectrum from the altimeter wave height. The analyzed wave spectrum may be cross validated with the synthetic aperture radar image spectrum. Implications for improvement of, for example, the Seasat scatterometer algorithm are pointed out.
Current response to wind in the Chukchi Sea: A regional coastal upwelling event
Authors: Johnson Walter R.
Published: February 2008

A cruise was conducted on the NOAA ship Oceanographer in August and September 1986 in the Chukchi Sea. Four short‐term moorings were instrumented with sediment traps and Aanderaa RCM4 current meters. The currents at the three moorings near the Alaskan coast indicate a reversal of the normal northeastward flow to southwestward. This reversal was produced by wind, which had begun to blow from the east‐northeast at up to 4.2 m s−1. The temperature time series indicated that upwelling colder water was moving upward near the coast. The nearshore mooring had the largest amplitude variation of currents and the largest temperature variation. The amplitude of the reversal decreased offshore. The station farthest from the coast was near the ice edge and on the other side of Barrow Canyon and a subsea bank. The flow there was consistently toward the east and is only poorly related to the wind. Water mass analysis was conducted using two techniques; the first was a traditional TS diagram method, and the second was a cluster analysis on TS pairs for the surface and near‐bottom waters. Based on the shapes of the TS curves and their positions on the TS diagrams, a map of the water masses was constructed. A cluster analysis was performed on the surface TS pairs from each station and separately for the bottom TS pairs. For both of these techniques, the line separating the groupings follows the temperature contours (5° at the surface and 4° at the bottom) and the bottom salinity contours (32.5 PSU).
The under‐ice thickness distribution of the Arctic Basin as recorded in 1958 and 1970
Authors: McLaren Alfred S.
Published: February 2008

The USS Nautilus (SSN‐571) was the first vessel to cross the Arctic Basin via the north pole in early August 1958. During this expedition, almost continuous acoustic under‐ice thickness profiles were recorded. In August 1970 USS Queenfish (SSN‐651) retraced Nautilus' route, yielding the only duplicate transect of under‐ice topography across the Arctic Basin. Comparisons of the statistical analysis of the under‐ice draft measurements obtained through use of wide‐beam and narrow‐beam and narrow‐beam only acoustic profilers by Nautilus and Queenfish, respectively, along the coincidental route are presented. Geographic areas found to have distinct under‐ice characteristics and ice composition are identified. The under‐ice statistics of both cruises are considered in relation to representative field observations and modeling results of other researchers. Principal findings are (1) Nautilus recorded generally more severe ice conditions within the Canada Basin than did Queenfish 12 years later; overall mean drafts were 3.08 and 2.39 m, respectively, (2) the under‐ice topography becomes progressively more severe when proceeding from the Canadian to the Eurasian side of the Arctic Basin, (3) the Canada Basin may contain the most moderate under‐ice topography and the greatest number of open water and refrozen polynyas and leads within the central Arctic Basin, (4) the Makarov and Amundsen basins and the Arctic mid‐ocean ridge may contain some of the most severe under‐ice topography within the Arctic Basin, and (5) the present study indicates an overall Arctic Basin mean of 3–4% open water/new ice (less than 30 cm) in summer.
The role of the Hessian matrix in fitting models to measurements
Authors: Thacker William Carlisle
Published: February 2008

A numerical model can be fit to data by minimizing a positive quadratic function of the differences between the data and their model counterparts. The rate at which algorithms for computing the best fit to data converge depends on the size of the condition number and the distribution of eigenvalues of the Hessian matrix, which contains the second derivatives of this quadratic function. The inverse of the Hessian can be identified as the covariance matrix that establishes the accuracy to which the model state is determined by the data; the reciprocals of the Hessian's eigenvalues represent the variances of linear combinations of variables determined by its eigenvectors. The aspect of the model state that are most difficult to compute are those about which the data provide the least information. A unified formalism is presented in which the model may be treated as providing either strong or weak constraints, and methods for computing and inverting the Hessian matrix are discussed. Examples are given of the uncertainties resulting from fitting an oceanographic model to several different sets of hypothetical data.
Synoptic analysis and dynamical adjustment of GEOS 3 and Seasat altimeter eddy fields in the northwest Atlantic
Authors: De Mey Pierre , Ménard Yves
Published: February 2008

A synoptic analysis of the dynamic topography of the sea surface in a (531 km)2 domain including the sites of the Polymode Synoptic Dynamics Experiment and Local Dynamics Experiment is presented. This study is based on all available GEOS 3 and Seasat altimeter passes between May and August 1978 referenced to an altimetric mean sea surface. The altimeter synoptic time series are compared to in situ estimates calculated from Polymode expendable bathythermographs, conductivity‐temperature‐depth probes, deep floats, and current meters. Wherever the altimeter coverage is good, both analyses show remarkable agreement. The following results are obtained: (1) the altimeter/in situ correlation in 0.56 on average and never falls below 0.38, (2) the beginning of operations of Seasat in the first days of July 1978 has a dramatic effect on all indicators, (3) the variance for the altimeter estimate is no more than 9% higher on average than the variance of the in situ estimate, and (4) the daily mean sea level is measured to within 2.4 cm on average. A first guess of the four‐dimensional stream function was calculated using the altimeter estimate at the surface and an empirical extension scheme. The resulting field was specified as initial and boundary values of the Harvard quasi‐geostrophic model. It is shown that the assimilation makes the altimeter estimate converge toward the in situ estimate especially at the subthermocline levels where spatial correlations of 0.60 are achieved after 8 days. A simultaneous assimilation of the altimeter stream function and 1400‐m Polymode current meter stream function was also performed, with even better results.
Mechanics of flow over ripples and dunes
Authors: Nelson Jonathan M. , Smith J. Dungan
Published: February 2008

A simple predictive model for velocity and boundary shear stress fields over two‐dimensional bedforms of finite amplitude is presented herein, and the results of that model are compared with flume data. The production of a wake due to separation of the flow near the bedform crest and the subsequent interaction of this wake region with a developing nonuniform boundary layer downstream of the flow reattachment point are treated using the method of McLean and Smith (1986). Their model is modified in order to take account of the flow response to short‐wavelength obstructions more accurately and to include the effects of wakes from upstream bedforms as these wakes coalesce in the interior. The theory is also extended to include the region between the bedform crest and the reattachment point, thereby providing a spatially complete field of velocity and shear stress. Results of the calculations are found to be in good agreement with laboratory measurements taken over immobile bedforms using a laser‐Doppler velocimeter. Our finite amplitude flow model is shown to yield accurate predictions of the fluid dynamical effects ultimately responsible for the morphology of well‐developed natural bedforms.
Simulations of radiocarbon in a coarse‐resolution world ocean model: 2. Distributions of bomb‐produced carbon 14
Authors: Toggweiler J. R. , Dixon K. , Bryan K.
Published: February 2008

Part 1 of this study examined the ability of the Geophysical Fluid Dynamics Laboratory (GFDL) primitive equation ocean general circulation model to simulate the steady state distribution of naturally produced 14C in the ocean prior to the nuclear bomb tests of the 1950s and early 1960s. In part 2 we begin with the steady state distributions of part 1 and subject the model to the pulse of elevated atmospheric 14C concentrations observed since the 1950s. This study focuses on the processes and time scales which govern the transient distributions of bomb 14C in the upper kilometer of the ocean. Model projections through 1990 are compared with observations compiled by the Geochemical Ocean Sections Study (GEOSECS) in 1972, 1974, and 1978; the Transient Tracers in the Ocean (TTO) expedition in 1981, and the French INDIGO expeditions in 1985–1987. In their analysis of the GEOSECS 14C observations, Broecker et al. (1985) noted that much of the bomb 14C which entered the ocean's equatorial belts prior to GEOSECS accumulated in the adjacent subtropical zones. Broecker et al. argued that this displacement of bomb 14C inventories was caused by the wind‐driven upwelling and surface divergence in the tropics combined with convergent flow and downwelling in the subtropics. Similar displacements were invoked to shift bomb 14C from the Antarctic circumpolar region into the southern temperate zone. The GFDL model successfully reproduces the observed GEOSECS inventories, but then predicts a significantly different pattern of bomb 14C uptake in the decade following GEOSECS. The post‐GEOSECS buildup of bomb 14C inventories is largely confined to the subthermocline layers of the North Atlantic, the lower thermocline of the southern hemisphere, and down to 2000 m in the circumpolar region. A great deal of attention is devoted to detailed comparisons between the model and the available radiocarbon data. A number of flaws in the model are highlighted by this analysis. The Subantarctic Mode Waters forming along the northern edge of the circumpolar current are identified as a very important process for carrying bomb 14C into the thermoclines of the southern hemisphere. The model concentrates its mode water formation in a single sector of the circumpolar region and consequently fails to form its mode waters with the correct T‐S properties. The model also moves bomb 14C into the deep North Atlantic and deep circumpolar region much too slowly.
Oceanography of Nares Strait and northern Baffin Bay in 1986 with emphasis on deep and bottom water formation
Authors: Bourke Robert H. , Addison Victor G. , Paquette Robert G.
Published: February 2008

Temperatures and salinities were measured from northern Baffin Bay through Nares Strait to the southern edge of the Lincoln Sea in September 1986. The water properties and baroclinic circulation, in general, did not vary widely from earlier measurements, but close station spacing led to the recognition of the rapid rate of dilution of Arctic Intermediate Water as it debouches into deeper water from the southern end of Kane Basin. As a result, this intermediate water becomes a much less likely source for the deep and bottom waters of Baffin Bay. The formation of deep and bottom waters more likely is initiated by the formation of cold, saline water in winter in the shallows of eastern Smith Sound under the thin ice of the North Water. This water then undergoes multicomponent mixing with the waters en route from its origin to the deep basin of Baffin Bay.
Weddell Gyre: Temperature maximum stratum
Authors: Bagriantsev Nikolai V. , Gordon Arnold L. , Huber Bruce A.
Published: February 2008

At depths below 200 m, relatively warm, salty water spreads poleward from the Antarctic Circumpolar Current (ACC). This deep water mass is cooled, through interaction with the atmosphere, feeding the formation of dense bottom water which in turn influences much of the world ocean. Significant poleward spreading is accomplished within the Weddell gyre of the Atlantic sector of the southern ocean. An extensive data set obtained in the 1980s within the Weddell gyre provides a detailed view of the subsurface temperature maximum (T‐max) stratum defining the top of the deep water mass. The seasonal sea ice growth and retreat pattern reflects the T‐max distribution. There is evidence for subdivision of the T‐max into two cold centers, which may reflect secondary circulation features within the gyre. The primary inflow of warm deep water is derived from the southern edge of the ACC near 20°–30°E. The inflow spreads to the west along 65°S and is divided into smaller “pools” of relatively warm water west of Maud Rise with a cold feature directly over the rise, in a manner shown by models of topographically generated eddies. Variability of the T‐max stratum in the vicinity of Maud Rise arising from changes in the vigor of the large‐scale circulation may influence the regional vertical heat fluxes and be related to interannual variability of the sea ice cover and to the recurring polynya feature typical of the region.
Turbulence and entrainment in a buoyant surface plume
Authors: Luketina David A. , Imberger Jörg
Published: February 2008

In winter, tidal outflow from the Leschenault Estuary forms a buoyant sheared stratified plume in Koombana Bay on each outgoing tide. The turbulence in the plume and the local entrainment into the plume are examined based on measurements collected in an intensive field program. Coherency measurements between horizontally separated sensors are used to examine the anisotropy of the turbulence. The effects of viscosity and buoyancy on the turbulence are studied using an activity diagram which is also an indicator of the mixing efficiency and total mixing. The Osborne‐Cox model is tested and found to be valid in the plume.
Cesium 137 and lead 210 in alpine lake sediments: Measurements and modeling of mixing processes
Authors: Pourchet Michel , Pinglot Jean F. , Mélières Marie A.
Published: February 2008

Simulations of 137Cs distribution in lake sediment cores are presented for slow and rapid mixing processes, for different mixing layers, and for different sampling thicknesses. A parameter characterizing the profile distribution is introduced. Interpretation of the 137Cs profiles of seven alpine lakes is given using those simulations leading to an estimate of the sedimentation rate, the type of mixing and the thickness of the mixing layer. These parameters are compared with those obtained independently from 210Pb profiles in three of the lakes. Agreement is satisfactory. The lakes we studied have similar features. In most cases the lakes have mixing which can be approximated by a rapid diffusion process, sedimentation rate of the order of 0.01 g cm−2 yr−1, and a mixing layer of the order of 2–4 cm.
Particulate new nitrogen fluxes in the Sargasso Sea
Authors: Altabet Mark A.
Published: February 2008

Seasonal variations of up to sevenfold in the flux of new nitrogen as sinking particles are observed at a site near Bermuda between January 1986 and May 1988. Maxima occur in the late winter with minima in the late fall. While this particle flux from the euphotic zone depends on levels of suspended particulate nitrogen (PN), NO3− flux modulated by seasonal variations in the magnitude of vertical transport appears to be the ultimate forcing. However, wintertime increases in euphotic zone NO3− concentration are not required for the high particle fluxes observed for this season. The gradient in NO3− concentration at 100 m is inversely correlated with particle flux resulting in the annual amplitude in new nitrogen flux to be well below the amplitude in vertical transport parameterized as a vertical eddy diffusivity. Yearly flux of sinking PN is 0.2 mol m−2 yr−1, one third the highest estimate reported for new nitrogen flux in this region. The downward mixing of suspended PN may make a significant contribution to the downward flux, particularly in the winter. Its inclusion raises the annual new nitrogen flux to 0.33 mol m−2 yr−1. This estimate is most likely conservative since sporadic events may not have been adequately sampled and the downward flux of nonparticulate forms of nitrogen has not been taken into account.
Observations and model of sediment transport near the turbidity maximum of the upper Saint Lawrence Estuary
Authors: Hamblin P. F.
Published: February 2008

The need to investigate the role of suspended sediments in the transport and fate of chemical contaminants in the St. Lawrence estuary has led to the measurement of profiles of suspended sediments, horizontal current, temperature, and salinity at an anchor station approximately 60 km downstream from the turbidity maximum. Hourly profiles over nearly three semidiurnal tidal cycles reveal peaks of suspended sediment concentration following maximum flood and ebb currents at the bottom, whereas near the surface there is only one maximum in suspended sediment concentrations per tidal cycle. Observations of the distributions of suspended sediment and its horizontal flux suggest that local resuspension is the controlling factor at the measurement site. This study demonstrates that landward sediment flux in the lower layer is maintained by the ebb‐flood asymmetry mechanism described by Dronkers (1986) and by the asymmetry in vertical mixing due to fluctuations in stratification related to the intrusion of the salt wedge. The latter mechanism is explored in detail by means of a vertical transport model for fine‐grained newly deposited sediments. The model employing standard prescriptions for mixing and resuspension results in the best match between simulated and observed sediment distributions for a particle sinking velocity of 3×10−4 m/s. This settling rate corresponds to a mean particle size of 15 μm, which compares closely with the average observed particle size of 10 to 20 μm (Krank, 1979).
Meridional variability of turbulence through the equatorial undercurrent
Authors: Peters H. , Gregg M. C. , Toole J. M.
Published: February 2008

Four meridional transects of the turbulent dissipation rate ε and of shear were taken across the equator along 140°W in late 1984. Averaging the upper 110 m of the transects, we found that the vertically averaged ε as well as shear showed maximum values within ±1° latitude. Because a strong diurnal cycle of ε aliased the measurements, we cannot extract spatial variations with good accuracy. However, when ε is separated into the contributions from the thermocline and from the surface mixed layer, consistent patterns emerge. In the thermocline, ε was strongest where the Richardson number was lowest, i.e., near the equator—consistent with the production of turbulence by the shear of the undercurrent. In the surface mixed layer near the equator, ε was usually larger than predicted by similarity scaling, but the discrepancy varied with the diurnal cycle of shear. When the mean shear was low, after prolonged mixing by nighttime convection, ε was close to the similarity scaling. We therefore attribute the excess dissipation to shear production.
Direct measurements of the flow of western Mediterranean deep water over the Gibraltar sill
Authors: Pettigrew Neal R.
Published: February 2008

The Gibraltar Pilot Experiment was carried out in June 1984 in order to obtain preliminary moored current measurements in the vicinity of the sill in the Strait of Gibraltar. Bottom‐mounted Doppler current measurements and moored temperature measurements from this experiment show directly the westward transport of Western Mediterranean Deep Water (WMDW) over the sill. The net westward transport of WMDW over the sill is roughly estimated to be 0.2×106 m3 s−1.