bluuebunny/crossref_metadata_2025 huggingface dataset:
Time evolution of surface chlorophyll patterns from cross‐spectrum analysis of satellite color images
Authors: Denman Kenneth L. , Abbott Mark R.
Published: February 2008

Sequences of coastal zone color scanner (CZCS) images from the offshore region adjacent to Vancouver Island, Canada, have been analyzed to estimate the time rate of decorrelation of surface phytoplankton chlorophyll pigment patterns. In these high‐latitude, high‐pigment areas, CZCS‐derived pigment estimates were lower than those obtained from ship samples by about a factor of 3, their frequency distributions were skewed in opposite directions, and subareas of the images often showed a discontinuity in the frequency distribution at a concentration of 1.5 mg m−3, where the algorithm changes CZCS bands. We selected cloud‐free subareas that were common to several images separated in time by 1–17 days. Image pairs were subjected to two‐dimensional auto spectrum and cross‐spectrum analysis in an array processor, and spectra of squared coherence were formed. The squared coherence estimates for several wave bands were plotted against time separation, in analogy with a time‐lagged cross correlation function. Threshold levels for significant coherence were estimated from many realizations of squared coherence calculated for pairs of synthetic random uncorrelated fields with specified power law behavior κ−1.5, near the observed range κ−1.5–κ−2. For wavelengths of 50–150 km, significant coherence is lost after 7–10 days, and for wavelengths of 25–50 km, significant coherence is lost after 5–7 days; in both cases offshore regions maintain coherence longer than coastal regions. For wavelengths of 12.5–25 km, only the offshore regions maintained coherence after 1 day, but that was clearly lost after the next time separation of 6 days. The implication for the formation of monthly average large‐scale surface maps to estimate open ocean productivity (e.g., Esaias et al., 1986) is that all mesoscale patterns (<150‐km length scale) will not be resolved.
Empirical orthogonal function analysis of advanced very high resolution radiometer surface temperature patterns in Santa Barbara Channel
Authors: Lagerloef Gary S. E. , Bernstein Robert L.
Published: February 2008

Empirical orthogonal function (EOF) analyses of a yearlong, advanced very high resolution radiometer (AVHRR) sea surface temperature (SST) data set from the Santa Barbara Channel describe a spatial and temporal variability which is consistent with in situ measurements. The analyses combine an unconventional decomposition of the spatial variance with a more conventional analysis of temporal variance modes to identify features in the AVHRR data such as fronts, eddies, and annual temperature cycles. The methodology has practical applications in the EOF analysis of remote sensing data which are more densely sampled in space than in time. The first mode (60%) of the spatial variance shows a persistent front aligned northwest‐southeast in the west central channel, which separates warmer surface water to the east from colder waters off Point Conception to the west. The eigenvector time series shows a seasonal modulation, peaking in early summer, consistent with in situ measurements of temperature gradient and current. The second spatial variance mode (7.7%) suggests an eddylike structure. The conventional EOF decomposition of temporal variance yields a heavily dominant first mode (91.6% variance) representing the basin‐wide seasonal warming and cooling. This mode peaks in early autumn and lags by 90 days the first‐mode time series revealed by the spatial variance analysis. The first temporal variance mode has little spatial variation in the eigenvectors, while the second and third modes show more spatial structure. The second and third temporal variance modes are extremely low energy (2.5% and 1.1%) and do not reveal statistically significant information on the frontal and eddy patterns evident in the AVHRR and in situ data.
Variability of surface pigment concentrations in the South Atlantic Bight
Authors: McClain Charles R. , Yoder James A. , Atkinson L. P. , Blanton J. O. , Lee T. N. , Singer J. J. , Muller‐Karger Frank
Published: February 2008

A 1‐year time sequence (November 1978 through October 1979) of surface pigment images from the South Atlantic Bight (SAB) was derived from the Nimbus 7 coastal zone color scanner. This data set is augmented with in situ observations of hydrographic parameters, freshwater discharge, sea level, coastal winds, and currents for the purpose of examining the coupling between physical processes and the spatial and temporal variability of the surface pigment fields. The SAB is divided into three regions: the east Florida shelf, the Georgia‐South Carolina shelf and the Carolina Capes. Six‐month “seasonal” mean pigment fields and time series of mean values within subregions were generated. While the seasonal mean isopleths were closely oriented along isobaths, significant differences between seasons in each region were found to exist. These differences are explained by correlating the pigment time series with physical parameters and processes known to be important in the SAB. Specifically, summertime concentrations between Cape Romain and Cape Canaveral were greater than those in winter, but the opposite was true north of Cape Romain. It is suggested that during the abnormally high freshwater discharge in the winter‐spring of 1979, Cape Romain and Cape Fear were the major sites of cross‐shelf transport, while the cross‐shelf exchange during the fall of 1979 occurred just north of Cape Canaveral. Finally, the alongshore band of high pigment concentrations increased in width throughout the year in the vicinity of Charleston, but near Jacksonville it exhibited a minimum width in the summer and a maximum width in the fall of 1979.
Measurement and interpretation of surface roughness changes induced by internal waves during the Joint Canada‐U.S. Ocean Wave Investigation Project
Authors: Gotwols B. L. , Sterner R. E. , Thompson D. R.
Published: February 2008

During the Joint Canada‐U.S. Ocean Wave Investigation Project (JOWIP), video and laser slope meter observations of surface waves perturbed by solitonlike internal waves were obtained from the Canadian research ship Endeavour. Two instances were observed of resonant interaction between wind waves that had been turned through a large angle, and the advancing internal wave current. When the wind makes a relatively small angle with the internal wave crest, the effect of the internal wave current is to split the (unperturbed) wind wave spectrum into a resonant system propagating with the interval wave, and a weaker wave system propagating against it. This phenomenon was clearly observed in the measurements and qualitatively confirmed by theoretical calculations.
Synthetic aperture radar imaging of surface ship wakes
Authors: Lyden James D. , Hammond Robert R. , Lyzenga David R. , Shuchman R. A.
Published: February 2008

Synthetic aperture radar (SAR) images of moving ships often exhibit characteristic patterns associated with various wake phenomena. These features can be classified into three general categories which include (1) surface waves generated by the ship, (2) turbulent or vortex wakes, and (3) internal waves. In this paper an overview of the present status of SAR ship wake imaging is presented.
Calculation of radar backscatter modulations from internal waves
Authors: Thompson D. R.
Published: February 2008

Calculations of microwave backscatter from the ocean surface using the small‐perturbation method (first‐order Bragg approximation), a modified Kirchhoff approximation, and a two‐scale composite model derived from the modified Kirchhoff expression are described, and predictions of the modulation in the cross section induced by internal wave features are presented. The calculations demonstrate that the modified Kirchhoff approximation and the composite model yield equivalent cross sections if the cutoff wave number separating the long‐ and short‐wavelength regions in the composite model is chosen to be roughly one‐third the Bragg wave number. The interaction of internal waves and surface waves, assumed to modulate an equilibrium wind wave spectrum over wavelengths extending from about 0.1 to 10 m, results in significant perturbations to the L band (24 cm) and X band (3 cm) cross sections at incidence angles from 23° to 50°. These calculations indicate that the large modulations observed in X band synthetic aperture radar images of internal waves are due, at least in part, to the tilting of the X band Bragg scatterers by the internal‐wave‐modulated long‐wavelength surface waves.
Comparisons of simulated and actual synthetic aperture radar gravity wave images
Authors: Harger Robert O. , Korman Can E.
Published: February 2008

The synthetic aperture radar (SAR) images obtained with the Jet Propulsion Laboratory's L band system in the Tower Ocean Wave and Radar Dependence (TOWARD) oceanographic experiment are compared with images generated by a simulation program implementing a SAR ocean imaging model based on two‐scale hydrodynamic and electromagnetic scattering approximate models. A critically regarded test of a theory of SAR ocean imaging is its prediction of the best image focus dependence on surface motion. The primary means of comparison here is an estimate of the best focus parameter that uses a subimage cross‐correlation technique. The focus parameter estimates for both the actual and simulated images show (1) a reversal in sign with reversal in dominant longwave direction relative to the SAR direction, (2) a magnitude increase with an increase in magnitude of the angle between the dominent long wave and SAR axes, and (3) an independence of the altitude and also the range‐to‐velocity ratio. All equivalent velocity estimates are of the order of the dominant longwave phase velocity, normalized by the SAR vehicle velocity, and agree well qualitatively though there are some quantitative differences. These behaviors are in agreement with a related analytical prediction. The “visibility” of the long waves' SAR image artifacts, for this TOWARD data set, increased with increased altitude and also with increased range‐to‐velocity ratio for both the actual and simulated images, as was predicted by a related analysis. Agreement of spectral density estimates of the actual and simulated SAR images generally required an order of magnitude increase in the strength of the hydrodynamic interaction of the long and short waves. This may indicate, as have surface measurements in TOWARD and other experiments, that the two‐scale model does not fully describe this nonlinear interaction. With this interaction adjustment, a comparison of actual and simulated images using an alternative focus criterion showed very close agreement in some cases.
Inertial wind path and sea surface temperature patterns near the Gulf of Tehuantepec and Gulf of Papagayo
Authors: Clarke Allan J.
Published: February 2008

During the northern hemisphere winter the surface atmospheric pressure on the Gulf of Mexico side of Central America is often higher than that on the Pacific side. The resultant southward pressure gradient drives strong southward winds through a gap in the Sierra Madre mountain range at the Isthmus of Tehuantepec. The atmospheric jet is narrow in comparison with the relevant radius of deformation, so when it leaves the coast and flows out over the Gulf of Tehuantepec, it is an inertial jet. Therefore the jet is expected to be deflected into a clockwise inertial circle by the Coriolis force. A similar process should also occur for Central American atmospheric jets through mountain range gaps near the Gulf of Papagayo. Satellite measurements of sea surface temperature together with coastal wind data for the first 41 days of 1986 were examined to see if the expected wind path influenced sea surface temperature (SST) near the Gulf of Tehuantepec and the Gulf of Papagayo. Consecutive daily satellite images when the coastal southward Tehuantepec wind increased suggest that the initial development of cold surface water in a clockwise loop is due to the wind mixing the shallow thermocline surface water along its inertial path. A simple 1½‐layer analytical ocean model suggests, in agreement with observations, that the wind can drive surface quasi‐geostrophic currents of ∼1 m s−1. Since the wind path is circular, the wind actually generates a clockwise rotating ocean eddy with the approximate scale of the wind inertial circle. The satellite sea surface temperature measurements also suggest that the wind‐driven surface ocean current is unstable. In addition, stronger gradients in SST to the right of the expected wind flow are consistent with advection of sea surface temperature by ocean surface Ekman flow.
Sea height wave form in equatorial waves and its interpretation
Authors: Musman Steven
Published: February 2008

In a previous paper a variation in sea slope of 1.6±0.6 cm per degree of latitude between 2°N and 4°N in Seasat altimeter data was associated with the passage of two temperature crests between 113°W and 130°W in the equatorial Pacific. This work has been extended to include variations in sea height between 1.5°S and 9°N with arbitrary phase in the wave. A mean wave pattern was found with maximum amplitude of 5 cm near 6°N. When the analysis was repeated for a range of wave periods, two preferred values are selected by minimum residuals. A period of 38.6±2.2 days corresponds with the passage of two observed temperature crests. A period of 22.2±0.9 days includes both these two crests and a possible intermediate crest of smaller amplitude. Fluctuating velocities in geostrophic equilibrium with the mean wave height are combined with a mean zonal flow to derive streamlines in a coordinate system moving westward with the phase velocity of the mean wave. One streamline shows the trochoidal form of the temperature front at about the appropriate latitude. Velocity field dependent estimates of the latitude of the crest and the angle of the crest are made. Flow immediately north of this streamline exhibits clockwise gyres near each trough. I suggest that this is the basic pattern of the observed form and motions in the waves. Also, selected streamlines are used to compute idealized drifter trajectories and compared with observations.
The feasibility of measuring ocean pH by long‐range acoustics
Authors: Guoliang Jin , Worcester Peter F.
Published: February 2008

Low frequency (<1000 Hz) sound absorption in the ocean depends exponentially on ocean pH. While the absolute amplitudes measured in long‐range (order of 1000 km) acoustic transmission experiments depend in a complex manner on ocean structure and are difficult to interpret without ambiguity, measurements of differential absorption as a function of frequency contain significant information. For surface‐reflected rays (RSR) there is the additional factor of frequency‐dependent reflectivity. In principle, a comparison of transmitted and received acoustic spectra can be inverted to obtain ocean pH and surface roughness. Calculations performed with preliminary data from a 750‐km path in the North Pacific give qualitative agreement with theory (higher frequencies are attenuated more than lower frequencies) but are inadequate for quantitative comparisons (the experiment was not optimized for differential attenuation measurements). Our conclusion is that it is possible, but not easy, to obtain quantitative information on ocean pH and surface roughness from measurements of differential attenuation with frequency along resolved ray paths. To obtain an accuracy of 0.05 in pH with a 750‐km transmission at 550±100 Hz would require about 100 independent samples for the North Pacific (pH ≈ 7.7) and 60 independent samples for the Atlantic (pH ≈ 8.0) in order to achieve adequate averaging of amplitude fluctuations induced by internal waves and ambient acoustic noise.
Variability on the California shelf forced by local and remote winds during the Coastal Ocean Dynamics Experiment
Authors: Davis Russ E. , Bogden Philip S.
Published: February 2008

The coastal response to local and remote wind forcing is examined using observations from the Coastal Dynamics Experiment (CODE) which took place on the northern California coast during 1981 and 1982. The responses to local and remote forcing are determined from multivariate statistical cross‐spectral analysis covering periods from 12 to 2.5 days. Remote forcing is deduced from coastal sea level. This analysis leads to discovery of a mode of sea level variability, apparently a barotropic Kelvin wave, with much greater alongshore and across‐shelf scales than those of coastally trapped waves; there is no measurable response to this mode in coastal temperature or velocity. Coastally trapped waves are also evident and have responses over the shelf much as predicted by theories which account for bottom friction. The local response to wind differs markedly from coastally trapped waves, showing the across‐shelf flow responsible for upwelling, surface‐intensified temperature fluctuations caused by advection and mixing and even the alongshore pressure gradient partly responsible for the across‐shelf return flow which, on the largest scale, balances Ekman transport. Unfortunately, even the extensive CODE observations are inadequate to diagnose the dynamics of the across‐shelf flow or accurately determine the cross‐shelf heat flux supported by eddies and the fluctuating responses to local wind forcing.
Bar‐generating cross‐shore flow mechanisms on a beach
Authors: Roelvink J. A. , Stive M. J. F.
Published: February 2008

Random waves normally incident on a dissipative beach induce a variety of cross‐shore flows, such as asymmetric oscillatory flow, wave grouping‐induced long‐wave flow, breaking‐induced turbulent flow, and momentum decay‐induced undertow. These flows are identified, analyzed and hindcasted in a set of laboratory experiments with the aim of revealing the role of each of the flow mechanisms in the two‐dimensional case of bar generation on a beach.
Tidal transients in the bottom layer on a slope
Authors: Hotchkiss Frances S. , Mollo‐Christensen Erik
Published: February 2008

High‐velocity transients observed near a sloping bottom are explained as solitary internal waves at the interface of the bottom mixed layer; the mixed layer is formed by overturning of the boundary layer on the flooding tide. The waves occur during both ebb and flood and are excited by instability. A simple model of the bottom boundary layer, combined with simple criteria for instability and wave formation, yields parameter values in agreement with observations from the Hudson Submarine Canyon.
The flocculation of fine‐grained sediments in estuarine waters
Authors: Burban Pierre‐Yves , Lick Wilbert , Lick James
Published: February 2008

Experimental and theoretical investigations of the effects of salinity, fluid shear, and sediment concentration on the rates of aggregation and disaggregation of a fine‐grained sediment in estuarine waters were made. Experiments were performed in fresh water, in seawater, and in an equal mixture of fresh and sea waters. The applied shears ranged from 100 to 600 s−1, while the sediment concentrations ranged from 10 mg/L to 800 mg/L. From the experimental and theoretical results and for the above range of parameters, the following was demonstrated: (1) the steady state particle size distribution is independent of the manner in which the steady state is approached; (2) the median floc size decreases as the shear stress increases; (3) the median floc size decreases as the suspended sediment concentration increases; (4) particles flocculate faster in seawater than in fresh water; (5) flocs are smaller in seawater than in fresh water; (6) the sizes of flocs in estuarine waters seem to be weighted averages of the sizes of flocs in fresh and sea waters; (7) in order to explain the observed decrease in floc size as the sediment concentration increases, the analysis requires disaggregation due to three‐body collisions; (8) the theory does not require disaggregation due to fluid shear. For the present range of parameters, fluid shear seems to have a negligible direct effect on disaggregation, while collisions between particles (possibly due to shear but also due to differential settling and Brownian motion) are the dominant mechanism for disaggregation; and (9) the theory can accurately reproduce the experimental results of mean diameter versus time and of steady state particle size distribution.
Hydrographical evidence for an isopycnal intrusion corroborated by acoustic Doppler current profiling
Authors: Cederlöf Ulf E. B. , Lundberg Peter A. , Østerhus Svein
Published: February 2008

Repeated ADCP sections without acoustic bottom reference and based on the directly observed athwartship velocity component have been worked across the surface water mass of the Faroe Bank Channel. A pronounced inhomogeneity of the current field was observed at depths of around 300 m. This anomaly could be correlated to the hydrographic observations which were undertaken throughout the experiment, since the pertinent segment of the temperature‐salinity (T‐S) relationship showed the presence of a water mass with characteristics deviating from those normally encountered in this zone. The outcome of a set of extended dynamic calculations furthermore showed that over this range of depths the observed velocity structure could not be accounted for solely in terms of geostrophic motion.
Seasonal variations in a linear barotropic model of the North Pacific driven by the Hellerman and Rosenstein wind stress field
Authors: Greatbatch Richard J. , Goulding Allan
Published: February 2008

An efficient numerical model is used to solve the linear barotropic equations of motion with North Pacific bottom topography and seasonal wind forcing (that is, monthly mean wind stresses with the annual mean removed). The model domain extends from 10°S to 60°N and from 120°E to 100°W with 1°×1° resolution. The model‐predicted seasonal transport and sea level variations are described and compared with the available data. It is found that at the Tokara Strait, to the south of Japan, the model‐predicted seasonal transport variations are very similar to those of the observed annual cycle of sea level difference across the Strait, although the amplitude of the model response is less than that suggested by the data. Both model and data indicate enhanced flow of the Kuroshio through the Strait in summer with a minimum in that flow in the winter. This is 6 months out of phase with the seasonal transport variations predicted by the flat‐bottomed Sverdrup relation. The model response is very sensitive, however, to the smoothing of the topography, with additional peaks in transport being found in the spring and the fall when the smoothing is increased. This suggests that care in handling the topography is required if a general circulation model is to reproduce this seasonal cycle. Away from the western boundary, the model‐predicted transport variations are quite similar to those in a flatbottomed ocean. The model also shows some success at reproducing features of the observed annual cycle of sea level corrected for both atmospheric pressure variations and the steric density effect. An interesting feature of the results is that the model‐calculated sea level at the offshore edge of the coastal waveguide is remarkably similar all the way along the North American coast from the Aleutian Islands to California. This shows a peak in model sea level in the summer and a minimum in the winter and indicates that it is the seasonal fluctuations of the subtropical gyre which dominate the model response even in the northern latitudes. However, the amplitude of this signal is somewhat less than that observed at the coast which, as previous studies have shown, is strongly influenced by coastal effects. The importance of wind forcing of the coastal waveguide is also apparent in the simple model described here, as is demonstrated by comparing model‐calculated sea level at the coast with that at points offshore.
Meteorologically forced subinertial flows through the Strait of Gibraltar
Authors: Candela Julio , Winant Clinton D. , Bryden Harry L.
Published: February 2008

The observed transport due to subinertial flows, with periods ranging from days to a few months, through the Strait of Gibraltar reaches values up to 106 m3 s−1, and is well correlated with atmospheric pressure fluctuations over the Mediterranean Sea. This transport is barotropic and accounts for 84% of the variance observed at these frequencies. A second mode accounts for 12% of the variance and is characterized by a node (zero amplitude) located at the mean depth of the interface between Atlantic and Mediterranean waters. This baroclinic mode is modulated by the spring‐neap tidal cycle, corresponding to an increased exchange of waters between Atlantic and Mediterranean at neap tides. A simple model of the Mediterranean, which includes two basins and two straits, indicates that the system has two resonant frequencies at periods of 1.2 and 5.6 days. The 1.2‐day resonance corresponds to the eastern and western basins oscillating through the Strait of Sicily, while that at 5.6 days is the resonance of the Mediterranean Sea with the North Atlantic Ocean. A good approximation to the observed flows is obtained when this model is forced with the most energetic mode of the atmospheric pressure over the eastern and the western Mediterranean and a friction term is included to limit the flow through the Strait of Gibraltar. Restricting the flow through the Strait of Sicily does not improve the agreement between the model and the observations for the flows at Gibraltar.
Non‐Weibull behavior observed in a model‐generated global surface wind field frequency distribution
Authors: Erickson David J. , Taylor John A.
Published: February 2008

The frequency distribution of the surface wind speeds at each 4.5°×7.5° latitude‐longitude grid point produced by the National Center for Atmospheric Research community climate model is examined. A modified Kolmogorov‐Smirnov (KS) test was used to ascertain the extent of Weibull behavior for perpetual January and July model runs. The modified KS test has been applied to 400 days of both perpetual January and July model runs using a 0.5‐day time sample. A global map of the KS statistic for each of the January and July control runs has been generated. These maps are presented for both the ocean and land wind fields. For the January oceanic winds, 29.2% of the frequency distributions were non‐Weibull at the 95% confidence level. Over land, 35.4% of the wind speed frequency distributions were judged to be non‐Weibull. For the July wind speed data, 32.7% of the ocean data sets were judged to be non‐Weibull, while for winds over land, 30.2% were found to be non‐Weibull. Large areas of Eurasia experience non‐Weibull behavior during the January run. The main oceanic latitudinal zones associated with a non‐Weibull character are from 5° to 30° in each hemisphere, a feature that is most evident in the July run. The monsoonal winds over the Indian Ocean are almost entirely Weibull. The non‐Weibull character of surface wind field frequency distributions may explain, in part, the apparent inability of some surface wind data sets to drive an ocean circulation model through inexact wind forcing.
Contributions of the Siberian shelf polynyas to the Arctic Ocean intermediate and deep water
Authors: Martin Seelye , Cavalieri Donald J.
Published: February 2008

The role of the open water regions or polynyas in the winter ice pack over the Siberian continental shelves in the production of Arctic Ocean Intermediate and Deep Water is examined through use of a combination of passive microwave satellite data, coastal weather station data, and oceanographic data. The satellite data allow us to derive a time series of the open water areas of selected polynyas for the 1978–1982 winters. The polynyas which occur north of Svalbard, around Franz Josef Land, along the west coast and north end of Novaya Zemlya, and to the east of Severnaya Zemlya and the Taymyr Peninsula are the primary oceanographic contributors. The open water time series for these polynyas are used in combination with coastal weather station data to estimate the heat loss from each polynya. For the region north of Svalbard, which is historically known as Whalers Bay, the study shows that the heat loss is sufficient to provide the observed cooling of the West Spitsbergen Current. For the other polynyas the heat loss is used to compute the ice production rates and the oceanic salt flux. We then use the historic oceanographic salinities with the salt fluxes to estimate the contribution of each polynya to the Arctic Ocean water masses. If we assume that the salt contributes only to the Arctic Intermediate Water, our results show that the shelves are capable of producing about 20–60% of this water. Alternatively, if the salt contributes only to the deep water of the Eurasian Basin, then without consideration of the mixing of the bottom water with the Greenland and Norwegian Sea water, the contribution from the shelves yields a renewal time of about 100 years. Our results imply that there is insufficient water produced in the shelf polynyas to perform all of the roles that have been historically assigned to it.
Geomorphologic and sediment transport characteristics of the middle reach of the Bahia Blanca estuary (Argentina)
Authors: Perillo Gerardo M. E. , Sequeira Mario E.
Published: February 2008

The geomorphologic and sediment transport characteristics of the middle reach of the Bahía Blanca estuary (Argentina) are presented. A large intertidal sedimentary structure divides the curved‐funnel‐shaped reach into the Main channel, which includes the waterway, and the Vieja channel. The right margin of the study area is bordered by a low marsh/tidal flat complex. Sedimentological and low‐penetration seismic data show that the area is formed mostly by relict sediments of Pleistocene and Pliocene ages. Present‐day deposits are very thin and are concentrated on the left platform between the navigation route and the intertidal structure. Comparison of detailed bathymetric surveys separated by 6.75 years resulted in a net erosion for all the area of 2.4×106 m3. Assuming that the volume of sediments dredged from the navigation channel (510,000 m3 in 6 years) was originated on the erosional sectors of the study area, approximately 250,000 m3/yr are still exported from the reach. Net accumulation (up to 2 m) is concentrated on the left margin, while erosional zones (up to 4.5 m) are located on the right margin of the Main channel and the middle portion of Vieja channel. Sediment transport calculations from 15 current meter stations distributed in the area indicate a net bed load transport of sand‐sized material directed towards the mouth of the estuary. The results suggest that dynamically, the middle reach of the Bahía Blanca estuary does not behave as a meander as was expected from its general shape. Although a preliminary estimation may indicate that the dredged sediment may proceed from the right margin, the general circulation of the estuary indicates that all reaches interchange sediments with their adjacent sectors. Sediments deposited in the middle reach are coming from the erosional processes acting in the inner reach and on the flanks of tidal flat channels.
Atmospheric fluxes of lead, zinc, and cadmium from frequency domain deconvolution of sedimentary records
Authors: Christensen Erik R. , Osuna Juan L.
Published: February 2008

A frequency domain deconvolution method to extract past input fluxes of particle‐associated pollutants from their sedimentary records is developed. The method is based on the assumption of a mixing layer below which no further mixing takes place. From the latter historical layer a filtered time record is obtained. Also, from known steady state sedimentation parameters, a response to a unit impulse input is calculated by forward finite difference programs. The unknown input flux is then generated by taking Fourier transforms of the two time series, forming the ratio of the transforms, and applying the inverse frequency transform. The method has been tested for internal consistency and applied to Pb, Zn, and Cd in several sediment cores from Lake Michigan. The results are consistent with those obtained previously with a time domain method. However, at the present stage of development, the time resolution in the frequency method (≈1 year) is considerably better than in a general time domain method (≈15 years). The effects of deconvolution are to sharpen peaks, to generate peaks where none were present in the unprocessed record, and to shift the onset time of pollution towards more recent times. In most cases, there is a remarkable agreement between reconstructed input records and alternative source functions. For example, maxima around 1969 for Pb and Cd in deconvolved input records are confirmed by the atmospheric loading data for the Great Lakes region (Pb) and U.S. consumption figures (Cd).
Global mesoscale variability from the Geosat Exact Repeat Mission: Correlation with ocean depth
Authors: Sandwell David T. , Zhang Bohai
Published: February 2008

We have developed a new technique for extracting global mesoscale variability from satellite altimeter profiles having large radial orbit error (∼3 m). Long‐wavelength radial orbit error, as well as other long‐wavelength errors (e.g., tides, ionospheric‐atmospheric delay, and electromagnetic bias), are suppressed by taking the derivative (slope) of each altimeter profile. A low‐pass filter is used to suppress the short‐wavelength altimeter noise (λ<100 km). Twenty‐two repeat slope profiles are then averaged to produce a mean sea surface slope profile having a precision of about 0.1 μrad. Variations in sea surface slope, which are proportional to changes in current velocity, are obtained by differencing individual profiles from the average profile. Slopes due to mesoscale dynamic topography are typically 1 μrad (i.e., a 0.1‐m change in topography over a 100‐km distance). Root‐mean‐square (rms) slope variability as low as 0.2 μrad are found in the southeast Pacific, and maximum slope variations up to 6–8 μrad are found in major western boundary currents (e.g., Gulf Stream, Kuroshio, Falkland, and Agulhas) and Antarctic Circum‐polar Current (ACC) systems. The global rms variability map shows previously unknown spatial details that are highly correlated with seafloor topography. Over most areas, the rms slope variability is less than 1 μrad. However at mid‐latitudes, areas of higher variability occur in deep water (>3 km) adjacent to continental shelves, spreading ridges, and oceanic plateaus. Variability is low in shallower areas (<3 km). Along the ACC, the meso‐scale variability appears to be organized by the many shallow areas in its path. We do not see convincing evidence that variability is higher downstream from topographic protrusions. Instead, the areas of highest variability occur in the deep basins (>4km).
Interannual variability in phytoplankton pigment distribution during the spring transition along the west coast of North America
Authors: Thomas A. C. , Strub P. T.
Published: February 2008

A 5‐year time series of coastal zone color scanner imagery (1980–1983, 1986) is used to examine changes in the large‐scale pattern of chlorophyll pigment concentration coincident with the spring transition in winds and currents along the west coast of North America. The data show strong interannual variability in the timing and spatial patterns of pigment concentration at the time of the transition event. In both 1980 and 1981 a large increase in pigment concentration occurs (from 0.5 mg m−3 to >3.0 mg m−3 in 1980 and from 0.3 mg m−3 to >1.5 mg m−3 in 1981). In these years, as well as in 1982, the increase takes place between ≈33°N and ≈41°N, but it occurs in a region centered 300 km offshore in 1980 and within 300 km of the coast in 1981 and 1982. These changes are in contrast to 1983 and 1986, when virtually no change in pattern or concentration is associated with the transition. Calculations of light availability and in situ data suggest this region is nutrient limited even in the mid to late winter. Time series of pigment concentration and wind forcing for extended periods before and after the transition date show that increases in concentration are associated with strong wind mixing events prior to the transition and with the onset of southward wind stress (upwelling) at the time of the transition. Interannual variability in the response of pigment concentration to the spring transition appears to be a function of spatial and temporal variability in vertical nutrient flux induced by wind mixing and/or the upwelling initiated at the time of the transition. Interannual differences in the mixing regime are illustrated with a one‐dimensional mixing model.
Agulhas current variability and inshore structures off the Cape Province, South Africa
Authors: Goschen Wayne S. , Schumann Eckart H.
Published: February 2008

Satellite imagery over a 3‐year period is used to provide statistical data on the position of the Agulhas Current, frontal features, and meanders off the southeast coast of South Africa. Data recorded during a cruise undertaken over the continental shelf off Algoa Bay in May 1987 provided subsurface temperature data of a large frontal feature and is used to correlate in situ measurements with satellite imagery. It is found that the inshore thermal front of the Agulhas Current lay, on average, at the shelf break northeast of Algoa Bay and seaward of it to the west. Its thermal core was about 26 km further out. The Agulhas Current influences the ocean structures over the shelf area by longer‐term Natal Pulses and shorter term warm surface frontal plume fluctuations. A case study of a rapidly propagating meander (average speed, 42 km/day) showed that an inshore plume formed a thin wedge over the cooler shelf water, and it illustrated that the surface signature as seen by satellite imagery does not necessarily correspond to the subsurface structure. The formation of a large shear‐edge eddy could also be followed and was possibly accentuated by a severe storm.
Residual circulation in shallow estuaries: 1. Highly stratified, narrow estuaries
Authors: Jay David A. , Smith J. Dungan
Published: February 2008

Long‐wave theory and simple turbulence closures have been used to show that three distinct types of circulation (highly stratified, weakly stratified, and partially mixed) arise in narrow, shallow estuaries from the finite amplitude of the tide and the interaction of stratification with vertical mixing. Each type has a different dominant process causing the vertical exchange of salt and fresh water on the flood, and each gives rise to a characteristic residual circulation. The tidal circulation in highly stratified shallow estuaries, described herein, is the result of a finite amplitude internal motion driven by the barotropic tide; shear instabilities at the interface are the major vertical exchange mechanism. The residual circulation is caused primarily by ebb‐flood asymmetry in interface position and thickness. A model based on our theoretical analysis shows that the interface thickness on flood is much less than the depth of flow up to a critical tidal amplitude at which the two‐layer flow is destroyed, and correctly predicts the tidal amplitude of the neap‐spring transition in the Columbia River Estuary. Tidal frequency internal wave motion is not found in weakly stratified and partially mixed estuaries. Because of the reduced tidal shear, shear instabilities are weak or absent, and the residual circulation assumes a very different character, as described in part 2.
Velocity and hydrographic structure of two Gulf of Mexico warm‐core rings
Authors: Cooper Cortis , Forristall George Z. , Joyce Terrence M.
Published: February 2008

Results from an extensive survey of two Gulf of Mexico warm‐core rings are presented. The data were taken during a 1‐week period in December 1983 using a shipboard acoustic Doppler current profiler (ADCP), expendable current probes (XCP), expendable bathythermographs (XBT), and a CTD. Two rings were observed—an older one in the western Gulf adjacent to the west Texas shelf, and one just separated from the Loop Current. The western ring was of order 200 km in diameter with peak currents of 1 m s−1 at the 100‐m level. Water properties were uniform and characteristic of the high‐salinity Caribbean subtropical underwater. The eastern ring was of order 300 km in diameter with peak currents of near 2 m s−1 at the 100‐m level. Water below 200 m is of Caribbean origin, while the surface waters show more variability suggesting some inflow of Gulf of Mexico common water. The evolution of the rings is also described based on the cruise measurements, satellite AVHRR, and other observations of opportunity.
An effect of sidewalls on waves in a wind wave channel
Authors: Longuet‐Higgins Michael
Published: February 2008

Many experiments on the generation of surface waves by wind have been carried out in laboratory wind wave channels with vertical sidewalls. In this note it is shown that surprisingly, the waves near the wall may be considerably steeper than those along the center line of the tank, by a factor of about 21/2.
Precise orbit computation for the Geosat Exact Repeat Mission
Authors: Haines Bruce J. , Born George H. , Rosborough George W. , Marsh James G. , Williamson Ronald G.
Published: February 2008

We have conducted an extensive investigation of orbit determination strategies for the Geosat Exact Repeat Mission (ERM). The goal of our studies is to establish optimum geodetic parameters and procedures for the computation of the most accurate Geosat orbits possible and to apply these procedures for the routine computation of Geosat orbits during the ERM for the following purposes: (1) to enhance the value of the Geosat oceanographic investigations by providing the user community with improved ephemerides, (2) to develop orbit determination techniques for the upcoming altimetric mission TOPEX/POSEIDON, and (3) to assess the radial orbit accuracy obtainable with recently developed gravity models. To this end, ephemerides for the entire first year of the ERM have been computed using the GEODYN II orbit program on the Cyber 205 supercomputer system at the Goddard Space Flight Center. The GEM‐T1 gravity solution was used in developing solutions in 25 separate 17‐day arcs. Estimated radial orbit errors have been reduced from a level of 3‐m root‐mean‐square (rms) with the operational orbits to about the 85‐cm level. Both the operational and precision ephemerides were produced from the same tracking data collected by the U.S. Navy's Operational Network, so most of this improvement is due to improved force and measurement modeling. Recently, a more accurate gravity field, GEM‐T2, has become available. Preliminary orbit tests performed with the GEM‐T2 gravity model, along with Geosat TRANET‐2 Doppler data that have recently been acquired, suggest that radial orbit accuracies of about 35 cm rms can be achieved for Geosat.
Links between South African summer rainfall and temperature variability of the Agulhas and Benguela Current systems
Authors: Walker N. D.
Published: February 2008

Close associations are identified between interannual sea surface temperature (SST) variability within the Agulhas Current system, the Agulhas Retroflection region, and the Benguela Current system and summer rainfall behavior of South Africa. The relationships between local ocean temperatures and summer rainfall are strongest when extreme global‐scale atmospheric events, i.e., Southern Oscillation high‐ and low‐index phases, are excluded from the time series. The results indicate that rainfall prediction efforts may benefit from a consideration of SST anomalies within the western portion of the south Indian Ocean, in combination with other meteorological indices. Surface winds, heat fluxes, and atmospheric boundary layer characteristics are investigated as ocean‐atmosphere coupling mechanisms. Easterly wind anomalies across the southwest Indian Ocean and over source regions of the Agulhas Current accompany and precede local oceanic “warm events” which co‐occur with increased rainfall. Whereas previous climatic research has revealed that intensification of the South Atlantic Anticyclone co‐occurs with wet periods over the summer rainfall region, this research has revealed an additional mode of climate variability involving enhanced atmospheric circulation within the southwest Indian Ocean. The simultaneous occurrence of strong easterly winds and a moister atmospheric boundary layer produces increased moisture flux convergence over tropical and subtropical regions of the eastern subcontinent. South of Africa, altered surface heat flux distributions enhance low‐level baroclinicity and instability within the marine boundary layer, optimizing conditions for the intensification of mid‐latitude frontal systems. These processes provide mechanisms which increase the likelihood of tropical‐temperate trough formation across southern Africa, the major rain‐producing system in summer.
Radium 228 distribution and mixing in San Nicolas and San Pedro Basins, southern California Borderland
Authors: Hammond Douglas E. , Marton Richard A. , Berelson Wiliam M. , Ku Teh‐Lung
Published: February 2008

Sediment profiles of 228Ra and 232Th and water column profiles of 228Ra are presented for San Nicolas and San Pedro Basins, located in the Southern California Borderland. These data are used to estimate fluxes of 228Ra from the sediments, the vertical eddy diffusivity (KZ) in the water column, and the upwelling velocity (wZ). The fluxes of 228Ra from sediments, estimated by integrating the downcore deficiency of 228Ra with respect to its parent nuclide 232Th, are 335±115 atoms/m2s for San Nicolas Basin and 166±50 atoms/m2s for San Pedro Basin. San Nicolas Basin sediments have an excess of 228Ra in the upper 4 cm, but irrigation and diffusion create a 228Ra deficiency from 4 to 30 cm. San Pedro Basin sediments are anoxic and laminated, have a 228Ra deficiency above 6–12 cm depth, and have no surface excess of 228Ra. Basin waters appear to be well‐mixed horizontally below sill depth and in steady‐state with respect to 228Ra. Concentrations of 228Ra increase from sill depth to the bottom, ranging from approximately 1.0 dpm/100 kg to 2.4 dpm/100 kg in San Nicolas Basin and from 1.6 dpm/100 kg to 3.1 dpm/100 kg in San Pedro Basin. Estimates of KZ and wZ at several horizons in these basins are made by evaluating two steady state mass balance equations, one for 228Ra and one for heat. This approach is hindered by an inability to accurately define the temperature in waters flowing over the sills and the apparent failure of topography to completely restrict horizontal exchange in the upper part of the water column below sill depth in each basin. Upper limits for KZ are 23 cm2/s at 1700 m in San Nicolas and 2.8 cm2/s at 850 m in San Pedro Basin. Our best estimates for KZ and wZ for the deeper part of the basins are: KZ = 5–17 cm2/s and wZ = 50–150 m/y in San Nicolas Basin, and KZ = 1–3 cm2/s and wZ = 20–50 m/y in San Pedro Basin. The differences in KZ between basins reflect differences in the buoyancy gradients. Upwelling velocities increase from the bottom to the sill of each basin.
Glacial meltwater input to the Alaska Coastal Current: Evidence from oxygen isotope measurements
Authors: Kipphut George W.
Published: February 2008

Results of a study of the oxygen isotopic composition of coastal, pelagic, and fresh waters from the northern Gulf of Alaska region are presented. This study was undertaken to investigate whether isotopic tracers could be of use in determining the important freshwater inputs to the Alaska Coastal Current (ACC) and whether they could confirm the presence of the ACC in coastal waters west of Kodiak Island. The Alaska Coastal Current, the major coastal circulation feature of the northern Gulf of Alaska, can be distinguished from oceanic waters on the basis of its lower salinity at least as far west as Kodiak Island. This study adds significantly to the small amount of oxygen isotopic information available for the waters of this region. The isotopic results suggest that in late summer, glacial meltwater may provide a substantial portion of the total freshwater runoff into the ACC, and that the ACC does extend as far to the west as Unimak Pass.