45 research outputs found
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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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SEASOAR observations during a COARE surveys cruise, W9211B, 12 December 1992 to 16 January 1993
An international Coupled Ocean-Atmosphere Response Experiment (COARE)
was conducted in the warm-pool region of the western equatorial Pacific Ocean over a four-month period from November 1992 through February 1993 (Webster and Lukas, 1992). Most of the oceanographic and meteorological observations were concentrated in the Intensive Flux Array (IFA) centered at 1'45'S, 156000'E. As part of this experiment, the R/V Wecoma conducted three survey cruises; each cruise included measurements of the temperature, salinity and velocity distribution in the upper 300 m of the ocean, and continuous meteorological measurements of wind, air temperature, humidity, etc. Most of these measurements were along a butterfly pattern that was sampled repeatedly during the three COARE Surveys cruises, W9211A and W9211B,
and W9211 C
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Intensified Diurnal Tides along the Oregon Coast
Intensified diurnal tides are found along portions of the Oregon shelf (U.S. West Coast) based on analyses
of high-frequency (HF) radar surface current data and outputs of a 1-km resolution ocean circulation model.
The K₁ tidal currents with magnitudes near 0.07 m s⁻¹ over a wider part of the shelf (Heceta Bank complex;
44°–44.5°N), previously predicted by Erofeeva et al., are confirmed here by newly available HF radar data.
Intensified diurnal tides are also found along the narrow shelf south of Heceta Bank. In the close vicinity of
Cape Blanco (42.8°N), diurnal tidal currents (K₁ and O₁ constituents combined) may reach 0.3 m s⁻¹. Appreciable
differences in diurnal tide intensity are found depending on whether the model is forced with tides
and winds (TW) or only tides. Also, diurnal variability in wind forcing is found to affect diurnal surface
velocities. For the case forced by tides alone, results strongly depend on whether the model ocean is stratified
[tides only, stratified (TOS)] or not [tides only, no stratification (TONS)]. In case TONS, coastal-trapped
waves at diurnal frequencies do not occur over the narrow shelf south of 43.5°N, consistent with the dispersion
analysis of a linear shallow-water model. However, in case TOS, diurnal tides are intensified in that area,
associated with the presence of coastal-trapped waves. Case TW produces the strongest modeled diurnal tidal
motions over the entire Oregon shelf, partially due to cross-shore tidal displacement (advection) of alongshore
subinertial currents. At Cape Blanco, diurnal tidal variability dominates the modeled relative vorticity
spectrum, suggesting that tides may influence the separation of the alongshore coastal jet at that location.Keywords: Boundary currents, Dynamics, Topographic effects, Currents, Ocean dynamics, Fronts, Circulation/ Dynamic
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Spatial and Temporal Variability of the M2 Internal Tide Generation and Propagation on the Oregon Shelf
A 1-km-horizontal-resolution model based on the Regional Ocean Modeling System is implemented along the Oregon coast to study average characteristics and intermittency of the M₂ internal tide during summer upwelling. Wind-driven and tidally driven flows are simulated in combination, using realistic bathymetry, atmospheric forcing, and boundary conditions. The study period is April through August 2002, when mooring velocities are available for comparison. Modeled subtidal and tidal variability on the shelf are in good quantitative agreement with moored velocity time series observations. Depth-integrated baroclinic tidal energy flux (EF), its divergence, and topographic energy conversion (TEC) from the barotropic to baroclinic tide are computed from high-pass-filtered, harmonically analyzed model results in a series of 16-day time windows. Model results reveal several “hot spots” of intensive TEC on the slope. At these locations, TEC is well balanced by EF divergence. Changes in background stratification and currents associated with wind-driven upwelling and downwelling do not appreciably affect TEC hot spot locations but may affect intensity of internal tide generation at those locations. Relatively little internal tide is generated on the shelf. Areas of supercritical slope near the shelf break partially reflect baroclinic tidal energy to deeper water, contributing to spatial variability in seasonally averaged on-shelf EF. Despite significant temporal and spatial variability in the internal tide, the alongshore-integrated flux of internal tide energy onto the Oregon shelf, where it is dissipated, does not vary much with time. Approximately 65% of the M₂ baroclinic tidal energy generated on the slope is dissipated there, and the rest is radiated toward the shelf and interior ocean in roughly equal proportions. An experiment with smoother bathymetry reveals that slope-integrated TEC is more sensitive to bathymetric roughness than on-shelf EF.KEYWORDS: Continental shelf/slope, Tides, Internal waves, Hindcasts, North Pacific Ocean, Regional model
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SEASOAR and CTD observations during a COARE surveys cruise, W9211A, 8 November to 8 December 1992
An international Coupled Ocean-Atmosphere Response Experiment
(COARE) was conducted in the warm-pool region of the western equatorial
Pacific Ocean over a four-month period from November 1992 through
February 1993 (Webster and Lukas, 1992). Most of the oceanographic and
meteorological observations were concentrated in the Intensive Flux Array
(IFA) centered at 1°45'S, 156°00'E. As part of this experiment, three survey
cruises were conducted on the R/V Wecoma; each cruise included
measurements of the temperature, salinity and velocity distribution in the
upper 300 m of the ocean, and continuous meteorological measurements of
wind, air temperature, humidity, etc. Most of these measurements were along
a butterfly pattern that was sampled repeatedly during the three COARE
Surveys cruises, W9211A and W9211B, and W9211C.
Our primary objective was to measure zonal and meridional gradients
across the center of the IFA. We originally intended to sample along a larger
pattern (with diagonals of 200 km) at the beginning and end of each cruise,
and to sample a smaller pattern (diagonals of 100 km) as continuously as
possible through the main portion of each cruise. Early in W9211A, we found
that the smaller pattern was not large enough to span the actual positions of
the profiling current meter array, and that frequent deviations from our
initial choice of longitude would be necessary to avoid moorings and quasistationary
ships. We therefore abandoned our plan of two separate sampling
patterns, and instead chose one Standard Butterfly Pattern with a meridional
section along 156°06'W and a zonal section along 1°50'S, connected in the
southwestern and northeastern quadrants. Along this track, we measured the
upper-ocean temperature and salinity by means of a towed undulating
Seasoar vehicle (Figure 1) equipped with a SeaBird CTD system, while
underway at 7-8 knots. CTD casts were made at the beginning and end of each
tow, primarily to check calibration of the Seasoar sensors; additional CTD
casts were occasionally made along portions of the standard sections while
Seasoar was disabled. Water velocity along the ship's track was measured by
means of the ship-borne acoustic Doppler current profiler.
This report summarizes the Seasoar and CTD observations from
Wecoma's first COARE Surveys cruise, W9211A. It also provides a cruise
narrative, and a brief description of the data processing procedures
Atmospheric forcing of the Oregon coastal ocean during the 2001 upwelling season
Copyrighted by American Geophysical Union. Meteorological conditions during an intensive oceanographic observational program in May through August 2001 along the central Oregon coast are described and related to larger-scale and longer-term conditions. Southward wind stresses of 0.05-0.1 N m⁻² occurred roughly 75% of the time, with a sustained period of dominantly southward stress from mid-June through July. Wind variations were correlated with variations in the large-scale Aleutian Low and North Pacific High pressure centers; correlations with the continental Thermal Low were small. Intraseasonal oscillations in alongshore wind stress (periods near 20 days) correlated with the north-south position of the jet stream. These stress oscillations drove 20 day oscillations in upper ocean temperature, with a lag of roughly 5 days for maximum correlation and amplitudes near 4°C. The sum of sensible and latent air-sea heat fluxes was generally into the atmosphere through June, then weakly into the ocean thereafter, with fluctuations on synoptic timescales. Semidiurnal fluctuations in surface air temperature were observed at two northern moorings, apparently forced indirectly by nonlinear internal ocean tides. The diurnal cycle of wind stress was similar for both southward and northward wind conditions, with the diurnal alongshore fluctuation southward in the evening and northward in the morning. During southward winds the marine atmospheric boundary layer (MABL) was typically defined clearly by a strong temperature inversion, and a shallow stable internal boundary layer often formed within the MABL over cool upwelled waters, with surface air temperature roughly 1°C lower inshore than offshore. During northward winds, essentially no low-level temperature stratification was observed
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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
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Convectively Driven Mixing in the Bottom Boundary Layer
Closely spaced vertical profiles through the bottom boundary layer over a sloping continental shelf during relaxation from coastal upwelling reveal structure that is consistent with convectively driven mixing. Parcels of fluid were observed adjacent to the bottom that were warm (by several millikelvin) relative to fluid immediately above. On average, the vertical gradient of potential temperature in the superadiabatic (statically unstable) bottom layer was found to be −1.7 × 10⁻⁴ K m⁻¹, or 6.0 × 10⁻⁵ kg m⁻⁴ in potential density. Turbulent dissipation rates (ε) increased toward the bottom but were relatively constant over the dimensionless depth range 0.4–1.0z/D (where D is the mixed layer height). The Rayleigh number Ra associated with buoyancy anomalies in the bottom mixed layer is estimated to be approximately 10¹¹, much larger than the value of approximately 10³ required to initiate convection in simple laboratory or numerical experiments. An evaluation of the data in which the bottom boundary layer was unstably stratified indicates that the greater the buoyancy anomaly is, the greater the turbulent dissipation rate in the neutral layer away from the bottom will be. The vertical structures of averaged profiles of potential density, potential temperature, and turbulent dissipation rate versus nondimensional depth are similar to their distinctive structure in the upper ocean during convection. Nearby moored observations indicate that periods of static instability near the bottom follow events of northward flow and local fluid warming by lateral advection. The rate of local fluid warming is consistent with several estimates of offshore buoyancy transport near the bottom. It is suggested that the concentration of offshore Ekman transport near the bottom of the Ekman layer when the flow atop the layer is northward can provide the differential transport of buoyant bottom fluid when the density in the bottom boundary layer decreases up the slope
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SeaSoar CTD observations from the central Oregon shelf, cruise W9907C : 13-31 July 1999 : a component of the Prediction of Wind-Driven Coastal Circulation Project
The primary objectives of R/V Wecoma cruise W9907C were to: 1) collect threedimensional fields of temperature, salinity, and light absorption and attenuation using the towed, undulating vehicle SeaSoar; 2) collect 3-D fields of velocity using shipboard ADCP; 3) to make turbulence profiles along a single cross-shelf transect; and 4) locate, recover, and either redeploy or replace a NOPP mooring which had been damaged by a fishing trawler earlier in the season