89 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 April–September 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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Observations of near-inertial surface currents off Oregon: Decorrelation time and length scales
High-resolution (km in space and hourly in time) surface currents observed by an array of high-frequency radars off Oregon are analyzed to quantify the decorrelation time and length scales of their near-inertial motions. The near-inertial surface currents are dominantly clockwise with amplitudes of 9-12 cm s⁻¹. However, they appear asymmetric and elliptical as a result of counterclockwise inertial motions with magnitudes in a range of 2-5 cm s⁻¹. The decorrelation time and length scales are computed from the decay slope of the near-inertial peak and the spatial coherence in the near-inertial frequency band, respectively. Decorrelation time scales of clockwise near-inertial motions increase from 2 days nearshore (within 30 km from the coast) to 6 days offshore, and their length scales increase from 30 to 90 km seaward possibly due to coastal inhibition. The local spatial coherence has an exponentially decaying structure for both clockwise and counterclockwise rotations, and their phases propagate northwestward (offshore) for clockwise and northeastward (onshore) for counterclockwise rotations.Keywords: coastal inhibition,
near-inertial oscillations,
spectral analysis,
Mixed layer,
decorrelation time scales,
surface currents,
Internal wave spectrum,
Upper ocean,
Wind,
decorrelation length scales,
Gravity wave
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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
25–50 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 0.73–0.83 and standard deviations
of the differences of 11.6–12.6 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 6.4–9.5 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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Biological and physical ocean indicators predict the success of an invasive crab, Carcinus maenas, in the northern California Current
An introduced population of European green crabs Carcinus maenas was established in San Francisco Bay (California, USA) prior to 1989. Subsequently, their larvae were likely carried northward into the embayments of Oregon, Washington (USA), and British Columbia (Canada) by the unusually strong Davidson Current during the winter of the El Niño of 1997-1998. Since this colonizing event, green crabs in Oregon and Washington have persisted at low densities. In this study, we show that after the arrival of the strong founding year-class of 1998, significant recruitment to the Oregon and Washington populations has occurred, but only in 2003, 2005, 2006 and 2010. Warm winter water temperatures, high positive values of the Pacific Decadal Oscillation (PDO) and Multivariate ENSO (El Niño Southern Oscillation) indices in March, weak southward shelf currents in March and April, a late biological spring transition, and high abundance of subtropical copepods are all strongly correlated with strong year-classes. We hypothesize that northward transport of larvae from California by coastal currents during warm winters is the mechanism by which the larvae are delivered to the Pacific Northwest. Among the best indicators of northward flow (and green crab recruitment) were the date of ‘biological spring transition’, the sign of the PDO, and the biomass of southern copepod species, which indicate (1) stronger northward flow of coastal waters during winters, (2) relatively warm winters (sea surface temperature >10°C), which enable larvae to complete their development in the near-shore, and (3) coastal circulation patterns that may keep larvae close to shore, where they can be carried by tidal currents into estuaries to settle.Keywords: El Niño, California current system, Plankton community structure, Year-class strength, Alongshore currents, Recruitment, Pacific Decadal Oscillation, European green crabKeywords: El Niño, California current system, Plankton community structure, Year-class strength, Alongshore currents, Recruitment, Pacific Decadal Oscillation, European green cra
Two coastal upwelling domains in the northern California Current system
A pair of hydrographic sections, one north and one south of Cape Blanco at 42.9N, was sampled in five summers (1998–2000 and 2002–2003). The NH line at 44.6N lies about 130 km south of the Columbia River, and spans a relatively wide shelf off Newport, Oregon. The CR line at 41.9N off Crescent City, California, lies 300 km farther south and spans a narrower shelf. Summer winds are predominantly southward in both locations but the southward winds are stronger on the CR line. Sampling included CTD/rosette casts (to measure temperature, salinity, dissolved oxygen, nutrients, chlorophyll), zooplankton net tows and continuous operation of an Acoustic Doppler Current Profiler. We summarize and compare July-August observations from the two locations. We find significant summer-season differences in the coastal upwelling domains north and south of Cape Blanco. Compared to the domain off Newport, the domain off Crescent City has a more saline, cooler, denser and thicker surface mixed layer, a wider coastal zone inshore of the upwelling front and jet, higher nutrient concentrations in the photic zone and higher phytoplankton biomass. The southward coastal jet lies near the coast (about 20–30 km offshore, over the shelf) on the NH line, but far from shore (about 120 km) on the CR line; a weak secondary jet lies near the shelf-break (35 km from shore) off Crescent City. Phytoplankton tend to be light-limited on the CR line and nutrient-limited on the NH line. Copepod biomass is high (15 mg C m−3) inshore of the mid-shelf on both NH and CR lines, and is also high in the core of the coastal jet off Crescent City. The CR line shows evidence of deep chlorophyll pockets that have been subducted from the surface layer. We attribute these significant differences to stronger mean southward wind stress over the southern domain, to strong small-scale wind stress curl in the lee of Cape Blanco, and to the reduced influence of the Columbia River discharge in this region
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Influence of varying upper ocean stratification on coastal near-inertial currents
The influence of varying horizontal and vertical stratification in the upper layer ( inline image m) associated with riverine waters and seasonal atmospheric fluxes on coastal near-inertial currents is investigated with remotely sensed and in situ observations of surface and subsurface currents and realistic numerical model outputs off the coast of Oregon. Based on numerical simulations with and without the Columbia River (CR) during summer, the directly wind-forced near-inertial surface currents are enhanced by 30%–60% when the near-surface layer has a stratified condition due to riverine water inputs from the CR. Comparing model results without the CR for summer and winter conditions indicates that the directly wind-forced near-inertial surface current response to a unit wind forcing during summer are 20%–70% stronger than those during winter depending on the cross-shore location, which is in contrast to the seasonal patterns of both mixed-layer depth and amplitudes of near-inertial currents. The model simulations are used to examine aspects of coastal inhibition of near-inertial currents, manifested in their spatial coherence in the cross-shore direction, where the phase propagates upward over the continental shelf, bounces at the coast, and continues increasing upward offshore (toward surface) and then downward offshore at the surface, with magnitudes and length scales in the near-surface layer increasing offshore. This pattern exhibits a particularly well-organized structure during winter. Similarly, the raypaths of clockwise near-inertial internal waves are consistent with the phase propagation of coherence, showing the influence of upper layer stratification and coastal inhibition.The data used in the paper will be available from the authors upon request ([email protected]) to comply with the American Geophysical Union Publications Data Policy.Keywords: near-inertial currents, stratification, coastal river plume, wind transfer function, coherence, coastal inhibitio
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Evaluation of directly wind-coherent near-inertial surface currents off Oregon using a statistical parameterization and analytical and numerical models
Directly wind-coherent near-inertial surface currents off the Oregon coast are investigated with a statistical parameterization of observations and outputs of a regional numerical ocean model and three one-dimensional analytical models including the slab layer, Ekman, and near-surface averaged Ekman models. The
transfer functions and response functions, statistically estimated from observed wind stress at NDBC buoys
and surface currents derived from shored-based high-frequency radars, enable us to isolate the directly wind-forced
near-inertial surface currents. Concurrent observations of the wind and currents are crucial to evaluate
the directly wind-forced currents. Thus, the wind stress and surface current fields obtained from a regional
ocean model, which simulates variability of the wind and surface currents on scales comparable to those in
observations, are analyzed with the same statistical parameterization to derive the point-by-point transfer
functions and response functions. Model and data comparisons show that the regional ocean model describes
near-inertial variability of surface currents qualitatively and quantitatively correctly. The estimated response
functions exhibit decay time scales in a range of 3-5 days, and about 40% of the near-inertial motions are
explained by local wind stress. Among the one-dimensional analytical models, the near-surface averaged
Ekman model explains the statistically derived wind-current relationship better than other analytical models.Keywords: wind transfer function, directly wind-coherent near-inertial currents, near-inertial currents, surface current
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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 (∼85 km long) section of shelf off the central Oregon coast is wide enough to allow first‐mode 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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