48 research outputs found
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Structure of the coastal current field off Northern California during the coastal ocean dynamics experiment
For 74 days during the spring and summer upwelling seasons of 1981 and 1982, in conjunction with the Coastal Ocean Dynamics Experiment, profiles of upper ocean currents were collected in the waters over the shelf and slope off northern California using a shipboard Doppler acoustic log. These measurements provide detailed information on the spatial structure of the current field. Synoptic maps of near-surface currents often deviate substantially from classical two-dimensional wind-driven upwelling and indicate a close association between the complex temperature structures observed in satellite imagery and the presence of vigorous current structures including squirts (regions of intense seaward flow), eddies, and countercurrents. Well-defined examples of a squirt and of a countercurrent during a cessation of wind forcing (wind relaxation event) are examined in detail. Despite this complexity, the nearshore synoptic current field was found to be anisotropic, varying more rapidly cross shore than alongshore. The structure of the current averages and simple fluctuation statistics were largely as deduced in other upwelling areas from moored measurements. A surface-intensified equatorward jet was found in the average alongshore currents, with vertical shear exceeding -2 x 10^(-3) s^(-1) over much of the shelf. The core of the jet moved offshore south of Point Arena and was better resolved in along-isobath averages. At depths below 80 m the average alongshore flow reversed, giving way to a poleward undercurrent, strongest near the shelf break. The detailed structure of the inferred cross-shore circulation was found to be sensitive to the coordinate system selected. Using nominal (along-coast) coordinates, the average cross-shore current was found to be directed offshore within a surface layer which deepened with distance from the coast out to the shelf break, showed strong vertical shear only near the surface, and was divergent in the upper water column and convergent in the lower water column over the shelf. Current fluctuations about the local mean were strongly polarized along isobaths near the coast and became largely isotropic far from shore except offshore from Point Arena, where squirts were recurrently observed
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On the spatial structure of coastal circulation off Newport, Oregon, during spring and summer 2001 in a region of varying shelf width
A time series of hourly surface current maps in the shelf waters off Newport, Oregon,
was made during AprilâSeptember 2001 using five SeaSonde HF current mappers, during
Coastal Advances in Shelf Transport (COAST). The surface currents responded rapidly
to the changing winds, in repeated patterns that were strongly affected by bottom
topography. An equatorward current jet repeatedly formed in response to upwelling winds,
its strength, but not its trajectory, covarying with the meridional wind stress. Near Cape
Foulweather (44.8°N), where the shelf begins to widen, the jet rotates, weakens, and
trends offshore to the south first along, then across, the isobaths. Below Cape Foulweather,
inshore of the jet, a lee region of generally weak currents was commonly observed.
The equatorward jet core was most commonly observed near the 80-m isobath between
45.0°N and 44.4°N, but transited offshore between spring and summer over Heceta Bank.
At Newport (44.6°N), it was rarely observed less than 8 km from the coast. A second,
inshore, equatorward jet, previously unknown, was observed repeatedly south of Waldport
(44.4°N). Sustained downwelling wind episodes produced poleward currents, though
less responsively north of Cape Foulweather. Strongest poleward flow was generally
trapped near the coast. Surface currents were correlated with the northward wind, except
regionally far from shore over Heceta Bank, responding within half a day. Response to the
wind varies spatially, being intensified in the narrow shelf (northern) region. The
equatorward jet persists through periods of zero wind forcing. Coastal sea level covaries
with the meridional wind and the primary mode of ocean current response
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Central California Coastal Circulation Study : CTD observations, cruise 8401, February 1984
CTD observations were made over the continental shelf and upper
continental slope from the coast to approximately 60 km off central California
between latitudes 34°N and 37.5°N. The measurements were made by Raytheon
Service Company as part of the Central California Coastal Circulation Study
sponsored by the Minerals Management Service. The objectives of this 18-month
field program were to obtain a set of observations of the ocean water mass and
velocity fields and develop a detailed description of these fields and their
seasonal and shorter period variations. The ultimate goal is to assess the
impact of exploitation of offshore oil and gas resources of the outer
continental shelf region. This data report contains vertical profiles,
horizontal maps at selected depths, and vertical sections of temperature,
salinity, dynamic height and relative geostrophic velocity from a total of
124 CTD casts made between 31 January 1984 and 10 February 1984
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Estimates of sea surface height and near-surface alongshore coastal currents from combinations of altimeters and tide gauges
Present methods used to retrieve altimeter data do not provide reliable estimates of
sea surface height (SSH) in the nearshore region, resulting in a measurement gap of
25â50 km next to the coast. In the present work, gridded SSH fields produced by
Archiving, Validation, and Interpretation of Satellite Oceanographic data (AVISO) in the
offshore region are combined with coastal tide gauge time series of SSH to improve
estimation in that gap along the west coast of the United States in the northern California
Current System between 40° and 45°N and 123.8° and 126°W. To assess the increase
in skill provided by this procedure, the geostrophic alongshore currents, calculated from
the new SSH fields in the gap region, are compared to three in situ, nearshore current
measurements, resulting in correlation coefficients of 0.73â0.83 and standard deviations
of the differences of 11.6â12.6 cm/s, substantially improved from the AVISO-only results.
When the Ekman current components are estimated and added to the geostrophic
currents, comparisons to the 10 m deep acoustic Doppler current profiler velocities are
only slightly improved. The Ekman components make a more significant contribution
when compared to HF radar surface current measurements, providing correlations of
0.94 and standard deviations of the differences of 6.4â9.5 cm/s. These results represent a
dramatic improvement in the quality of the SSH fields and estimated alongshore
currents when additional, realistic SSH data from the coastal region are added.
Here we use coastal tide gauges to provide the additional SSH data but also discuss more
general approaches for altimeter SSH retrievals in coastal regions where tide
gauge data are not available
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Tidal currents on the central Oregon shelf : models, data, and assimilation
Measurements of tidal currents on the central Oregon shelf are available from several sources, including recent high frequency (HF) coastal radar and Acoustic Doppler Profiler (ADP) deployments, and historical current moorings. In this paper we use a generalized inverse (GI) approach to compare these data to, and then assimilate them into, numerical models for the barotropic tides. Harmonic analysis of the data in short time windows using a modified admittance approach reveals that tidal currents on the Oregon shelf are highly variable in time, and can contain significant baroclinic components. Data from the winter months, when waters on the shelf are only weakly stratified, are found to be most nearly barotropic and thus most reasonable for assimilation into the shallow water equations model. The various data sources are used in several different combinations for assimilation and validation. Forcing the prior forward model with normal flow open boundary conditions obtained from a regional barotropic inverse model results in semidiurnal barotropic currents that are consistent (within estimated error limits) with all available data. In contrast, diurnal currents on the shelf are very sensitive to details of the model configuration, and are significantly improved by data assimilation. Very similar solutions result from assimilation of either the HF radar or ADP data sets. The high sensitivity of the diurnal band currents can be understood dynamically in terms of trapped shelf waves. A short (âŒ85 km long) section of shelf off the central Oregon coast is wide enough to allow firstâmode barotropic shelf waves at the subinertial diurnal frequencies. This results in locally resonant large amplitude diurnal tidal currents that are very sensitive to details in the local forcing, and hence quite variable in time
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Acoustic Doppler current profiler observations during the coastal mixing and optics experiment : R/V Endeavor cruises from 14 August to 1 September 1996 and 25 April to 15 May 1997
We present velocity observations from a shipboard acoustic Doppler current profiler (ADCP) on RN Endeavor during cruises E9608 (14 August to 1 September 1996) and E9704 (25 April to 15 May 1997). The cruises were conducted as part of the Office of Naval Research Coastal Mixing and Optics Accelerated Research Initiative. The objective was to rapidly survey a region in the Middle Atlantic Bight south of Cape Cod, Massachusetts. The ADCP was an RD Instruments
hull-mounted 307-kHz unit. Data were collected nearly continuously during both
in a region about 80 km square around 40.5°N, 70.5°W. Vertical bin length was 4
depth range in open water was 200 m. To reference the velocities to earth
used bottom-tracking supplemented by differential global positioning system (GPS) navigation.
A GPS attitude system was also used, in combination with the ship's gyrocompass,
heading. This report describes the ADCP processing steps and presents the observed velocities.
In addition, we apply an empirical tidal model to estimate and then remove the
currents, and we present the resulting subtidal velocities
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Secondary circulation associated with a shelfbreak front
Evidence for secondary circulation associated with a shelfbreak front is obtained from a highâresolution, crossâshelf section of hydrographic, optical and velocity fields. Convergence in the bottom boundary layer on the inshore side of the front and subsequent upwelling into the interior is evident by a midâwater region of suspended bottom material emanating from the foot of the front and extending to within 35 m of the surface, 80 m above bottom. Downwelling on the offshore side of the front in the upper water column is inferred from a 20âm downward bend of the subsurface phytoplankton layer. These observations are in agreement with recent model predictions for secondary circulation near an idealized shelfbreak front. Convergence in measured crossâshelf velocity at the foot of the front is consistent with upwelling of bottom material detected there. An estimate of 9±2 m dayâ»Âč of upwelling on the inshore side of the shelfbreak front is obtained, implying a transit time from the bottom to the surface of 10â16 days
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Poleward flow off central California during the spring and summer of 1981 and 1984
Currents and winds measured over the continental shelf and upper continental slope during the first
half of 1984 are analyzed to determine the character of the flow off central California (Point Conception
to San Francisco). The mean flow was poleward from Point Conception to Point Sur, in opposition to
the equatorward wind stress. The mean flow was equatorward north of Point Sur. Fluctuations in
alongshore currents over the shelf were highly correlated with local winds everywhere except off Point
Conception, where currents were not only uncorrelated with wind but also poorly correlated with
currents farther north. North of Point Buchon there is evidence for poleward propagation of shelf
current variability at 175-200 cm/s. The correlation between currents and local wind forcing dropped
considerably beyond the shelf break, only 10-15 km offshore from the shelf moorings. During extended
periods of weak equatorward winds, the poleward shell flow south of Point Sur spreads farther offshore
over the continental slope. A three week period of calm winds in July 1984 resulted in a 100-km-wide
tongue of poleward flow extending 300 km along the California coast. Similar, but somewhat weaker,
poleward surface flow occurred during the same period in 1981; a 2- to 3-week period of calm winds in
late June and early July 1981 resulted in a 100-km-wide tongue of poleward flow extending at least 150
km along the California coast. In contrast to the poleward flow on the shelf, which appears to be normal
during spring and summer, the poleward surface flow observed over the slope in July 1981 and 1984 is
unusual, based on historical hydrographic surveys off the central California coast. Poleward surface flow
over the continental slope occurs seasonally in the winter but is not generally observed after February
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Assimilation of surface velocity data into a primitive equation coastal ocean model
A data assimilation system (DAS) of the windâdriven, mesoscale shelf circulation off the Oregon coast is developed. The DAS assimilates lowâpass filtered surface velocity measurements, obtained from landâbased highâfrequency coastal radar arrays, into a primitive equation coastal ocean model using a sequential optimal interpolation scheme. Inhomogeneous and anisotropic estimates of the forecast error covariances required for the assimilation are assumed to be proportional to typical crossâcorrelations between modeled variables. These correlations are estimated from an ensemble of model simulations for 18 different summers. Similarly, the observation error covariances are assumed to be proportional to the actual covariances of the observations. A timeâdistributed averaging procedure (TDAP) that effectively lowâpass filters the model forecast for comparison with the observations and introduces the corrections to the model state gradually over time is used in order to overcome problems of data compatibility and initialization. The correlations between direct subsurface current measurements and subsurface currents obtained from modelâonly and assimilation experiments for the summer of 1998 are 0.42 and 0.78, respectively, demonstrating the effectiveness of the DAS. Our estimates of the error covariances are shown to be appropriate through a series of objective statistical tests. Analysis of the term balances of the model equations show that the dominant modeled dynamical balances are preserved by the DAS and that uncertainties in the spatial variability of the wind forcing are likely to be one source of model error. By varying the relative magnitudes of the estimated forecast and observation error covariances the DAS is shown to be most effective when approximately 80% of the analysis is made up of the model solution
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Evaluation of a coastal ocean circulation model for the Columbia River plume in summer 2004
Realistic hindcast of the Columbia River estuarine-plume-shelf circulation in summer
2004 using the Regional Ocean Modeling System nested within the Navy Coastal Ocean
Model (NCOM) is quantitatively evaluated with an extensive set of observations. The
model has about equal skill at tidal and subtidal properties. Tidal circulation and water
properties are best simulated in the estuary, which is strongly forced and damped, but
worst on the shelf. Subtidal currents are again best in the estuary. However, subtidal
temperature and salinity are best simulated in the surface waters on the shelf, even inside
the river plume. A comprehensive skill assessment method is proposed to evaluate the
cross-scale modeling system with a focus on the plume. The model domain is divided into
five dynamical regions: estuary, near- and far-field plume, near surface and deep layers. A
skill score is obtained for each region by averaging the skills of different physical variables,
and an overall skill is obtained by averaging the skills across the five regions. This
weighting metric results in more skill weight per unit volume in the near surface layer where
the plume is trapped and in the estuary. It is also demonstrated, through model/data
comparison and skill assessment, that by nesting within NCOM, some important remote
forcing, e.g., coastal trapped waves, are added to our model; on the other hand, some biases
are also received. With a finer grid and more realistic forcing, our regional model improves
skill over a larger-scale model in modeling the shelf-plume circulation