J. S. Allen

Some Three-Dimensional Characteristics of Low-Frequency Current Fluctuations near the Oregon Coast

Abstract An analysis is presented of the low-frequency fluctuations [ω<0.6 cycle per day (cpd)] of the currents near the Oregon coast, based on the 1972 and 1973 measurements from moored current meters in CUE-1 and CUE-2. Let u and v denote the eastward (approximately onshore) and northward (approximately alongshore) components of the currents. The mean alongshore velocity v has the structure of a baroclinic coastal jet, whose maximum speed occurs near the surface at a distance of about 15–20 km from the shore, whereas the fluctuating part of v has the structure of a roughly barotropic coastal jet whose maximum occurs very near (<4 km) the shore. The standard deviation of v is approximately depth-independent whereas that of u decreases with depth. As one approaches the coast, the standard deviation of u decreases whereas that of v rises steeply, consistent with the behavior expected of coastally trapped wave motion. A scatter plot of the velocity fluctuations in a hodograph plane indicates that the fluctu...

Swash zone sediment suspension and transport and the importance of bore-generated turbulence

A study of swash zone sediment transport was conducted at Gleneden Beach, Oregon during February 25–28, 1994. The data collected included suspended sediment concentration (SSC), sea surface elevation, and velocity (initially 4 and 8 cm above the bed) at three cross-shore locations within the swash zone spanning high tides. Ensemble averages of 6, 9, and 12 s duration swash events showed that the uprush suspension was high, concentrated in the leading edge, and nearly vertically uniform above the lower 1–2 cm of the water column. Shortly after the sensors were inundated by run-up, the sediment rapidly settled out of the water. During flow reversal the SSC was small but increased again in the backwash. Backwash vertical profiles were markedly different from uprush profiles with much of the suspension being confined to very near the bed where strong vertical gradients in SSC existed. These marked differences show that the backwash is not simply the reverse of the uprush, implying significant differences in the underlying fluid dynamics and sediment transport mechanisms. Backwash sediment suspension increased with flow duration. However, ensemble-averaged SSC profiles of varying duration showed that the backwash concentrations were not consistent at the same temporal phases, which suggests that water depth, in addition to flow duration, may be a controlling factor. Strong cross-shore gradients in SSC suggest that bore-derived turbulence may affect local sediment transport. Specifically, our data show this bore-generated turbulence (turbulent kinetic energy) directly influences local sediment suspension, hence, standard bed shear (Bagnold-type) sediment transport models may no longer be valid in the vicinity of the bore. In the vicinity of the bore a higher correlation between bore-generated turbulence and suspended sediment transport was found than between a Bagnold-type formulation and suspended sediment transport.

Modal Decomposition of the Velocity Field near the Oregon Coast

Abstract The low-frequency [ω<0.5 cycle per day (cpd)] current fluctuations at four depths in 100 m of waterhave been investigated for two stations on the continental shelf off the coast of Oregon. One station, DB-7,was maintained during the summer of 1972 as part of the Coastal Upwelling Experiment-1 (CUE-I), and theother station, Carnation, was maintained during the summer of 1973 as part of CUE-II. A decomposition ofthe north-south (almost alongshore) v and the east-west (onshore-offshore) u components of the current hasbeen performed in terms of two types of modal structures in the vertical direction: (i) dynamic modes determined by the separable solutions of the appropriate equations of motion, and (ii) empirical orthogonal modeswhich are the eigenvectors of the correlation matrix and depend only on the statistics of the data. For thealongshore currents, the standard deviation of the dynamic barotropic mode is found to be twice as large asthat of the first baroclinic mode. The barotropic part is foun...

Nonlinear shear instabilities of alongshore currents on plane beaches

Evidence for the existence in the nearshore surf zone of energetic alongshore propagating waves with periods O(100 s) and wavelengths O(100 m) was found from observations by Oltman-Shay et al. (1989). These oscillations have wavelengths that are much too short to be surface gravity waves at the observed frequencies. The existence and properties of the wave-like motions were found to be related to the presence, strength and direction of an alongshore current in the surf zone. Based on a linear stability analysis of a mean alongshore current with offshore scale O(100 m), Bowen & Holman (1989) described these fluctuations as unstable waves associated with a shear instability. Good agreement of wavelengths and wave speeds from observations and from predictions based on the most unstable linear mode was obtained by Dodd et al. (1992). The nonlinear dynamics of finite-amplitude shear instabilities of alongshore currents in the surf zone are studied here utilizing numerical experiments involving finite-difference solutions to the shallow water equations for idealized forced dissipative initial-value problems. Plane beach (i.e. constant slope) geometry is used with periodic boundary conditions in the alongshore direction. Forcing effects from obliquely incident breaking surface waves are approximated by an across-shore-varying steady force in the alongshore momentum equation. Dissipative effects are modelled by linear bottom friction. The solutions depend on the dimensionless parameter Q, which is the ratio of an advective to a frictional time scale. The steady frictionally balanced, forced, alongshore current is linearly unstable for Q less than a critical value Q C . The response of the fluid is studied for different values of AQ = Q C -Q. In a set of experiments with the alongshore scale of the domain equal to the wavelength 2π/k 0 of the most unstable linear mode, disturbances that propagate alongshore in the direction of the forced current with propagation velocities similar to the linear instability values are found for positive ΔQ. The disturbances equilibrate with constant amplitude for small ΔQ and with time-varying amplitudes for larger ΔQ. For increasing values of ΔQ the behaviour of this fluid system, as represented in a phase plane with area-averaged perturbation kinetic energy and area-averaged energy conversion as coordinates, is similar to that found in low-dimensional nonlinear dynamical systems including the existence of non-trivial steady solutions, bifurcation to a limit cycle, period-doubling bifurcations, and irregular chaotic oscillations. In experiments with the alongshore scale of the domain substantially larger than the wavelength of the most unstable linear mode, different behaviour is found. For small positive AQ, propagating disturbances grow at wavelength 2π/k 0 . If ΔQ is small enough, these waves equilibrate with constant or spatially varying amplitudes. For larger ΔQ, unstable waves of length 2π/k 0 grow initially, but subsequently evolve into longer-wavelength nonlinear propagating steady or unsteady wave-like disturbances with behaviour dependent on AQ. The eventual development of large-scale nonlinear propagating disturbances appears to be a robust feature of the flow response over plane beach geometry for moderate, positive values of AQ and indicates the possible existence in the nearshore surf zone of propagating finite-amplitude shear waves with properties not directly related to results of linear theory.

A Modeling Study of the Three-Dimensional Continental Shelf Circulation off Oregon. Part I: Model–Data Comparisons

Sixty-day simulations of the subinertial continental shelf circulation off Oregon are performed for a hindcast study of summer 1999. Model results are compared with in situ currents, high-frequency radar‐derived surface currents, and hydrographic measurements obtained from an array of moored instruments and field surveys. The correlations between observed and modeled alongshore currents and temperatures in water depths of 50 m are in excess of 0.8. A study designed to test the model’s sensitivity to different initial stratification, surface forcing, domain size, and river forcing demonstrates that surface heating is important, and that the model results are sensitive to initial stratification. An objective criterion for assessing the skill of a model simulation relative to a control simulation is outlined, providing an objective means for identifying the best model simulation. The model‐data comparisons demonstrate that temperature fluctuations off Newport are primarily in response to surface heating and that subsurface density fluctuations are controlled by the wind-forced circulation through salinity. Experiments with river forcing indicate that, in the vicinity of Newport, the Columbia River plume is typically greater than 15 km from the coast and is confined to the top few meters of the water column. Additionally, the model‐data comparisons suggest that the strongest upwelling occurs to the north of Newport where the continental shelf is relatively narrow and uniform in the alongshore direction. Part II of this study investigates the modeled three-dimensional circulation and dynamical balances. A numerical modeling study of the coastal ocean circulation off Oregon during the 1999 upwelling season is presented. The model results are compared with in situ velocity, temperature and salinity measurements, and high-frequency (HF) radar‐derived surface currents obtained during summer 1999 as a part of the Oregon State University (OSU) National Oceanographic Partnership Program (NOPP) project. The aims of this study are to assess the performance of the model, to identify the dominant physical processes, and to assess the model’s sensitivity to variations in initial stratification, surface forcing, model domain size, and river forcing. Processes that are of particular interest in this study include the response to wind forcing and the generation of the northward flow that is commonly observed off Newport (44.658N) over the

On the Effect of Bottom Friction on Barotropic Motion Over the Continental Shelf

Abstract Observations of the velocity fields over the continental shelf and slope off Oregon and off Peru have shown that there is a phase difference in the onshore-offshore direction, with the velocity fluctuations nearshore leading those offshore in time. It is shown here that the effects of bottom Ekman layer friction and cross-shelf depth variation combine to result in such a phase lag in a model for forced or free long barotropic continental shelf waves. The model also shows that bottom friction results in a smaller phase lag between the alongshore components of velocity and wind stress than that predicted by a frictionless model, a feature found in the observations off Oregon.

Modeling study of turbulent mixing over the continental shelf: Comparison of turbulent closure schemes

limitation is imposed. During upwelling-favorable winds, the majority of turbulent mixing occurs in the top and the bottom boundary layers and in the vicinity of the vertically and horizontally sheared coastal jet. Turbulent mixing in the coastal jet is primarily driven by shear-production. The near-surface flow on the inner shelf becomes convectively unstable as wind stress forces the upwelled water to flow offshore in the surface layer. During downwelling-favorable winds, the strongest mixing occurs in the vicinity of the downwelling front. The largest turbulent kinetic energy and dissipation are found near the bottom of the front. Turbulence in the bottom boundary layer offshore of the front is concentrated between recirculation cells which are generated as a result of symmetric instabilities in the boundary layer flow. INDEX TERMS: 4219 Oceanography: General: Continental shelf processes; 4568 Oceanography: Physical: Turbulence, diffusion, and mixing processes; 4255 Oceanography: General: Numerical modeling; 4279 Oceanography: General: Upwelling and convergences;

A Modeling Study of the Three-Dimensional Continental Shelf Circulation off Oregon. Part II: Dynamical Analysis

Abstract Sixty-day simulations of the subinertial continental shelf circulation off Oregon are performed for a hindcast study of summer 1999. In Part I, the model results are shown to compare favorably with in situ currents and hydrographic measurements obtained from an array of moored instruments and field surveys and high-frequency radar–derived surface currents. In this paper, the modeled three-dimensional, time-varying circulation and dynamical balances are analyzed, providing a detailed synoptic description of the continental shelf circulation off Oregon for summer 1999. The circulation is clearly wind driven and strongly influenced by alongshore variations in shelf topography. In the region of the coast where the alongshore topographic variations are small the upwelling circulation is consistent with standard conceptual models for two-dimensional across-shore circulation. In the regions where the alongshore topographic variations are greater, the upwelling circulation is highly three-dimensional. Ov...

The M2 Internal Tide off Oregon: Inferences from Data Assimilation

A linearized baroclinic, spectral-in-time tidal inverse model has been developed for assimilation of surface currents from coast-based high-frequency (HF) radars. Representer functions obtained as a part of the generalized inverse solution show that for superinertial flows information from the surface velocity measurements propagates to depth along wave characteristics, allowing internal tidal flows to be mapped throughout the water column. Application of the inverse model to a 38 km 3 57 km domain off the mid-Oregon coast, where data from two HF radar systems are available, provides a uniquely detailed picture of spatial and temporal variability of the M2 internal tide in a coastal environment. Most baroclinic signal contained in the data comes from outside the computational domain, and so data assimilation (DA) is used to restore baroclinic currents at the open boundary (OB). Experiments with synthetic data demonstrate that the choice of the error covariance for the OB condition affects model performance. A covariance consistent with assumed dynamics is obtained by nesting, using representers computed in a larger domain. Harmonic analysis of currents from HF radars and an acoustic Doppler profiler (ADP) mooring off Oregon for May‐July 1998 reveals substantial intermittence of the internal tide, both in amplitude and phase. Assimilation of the surface current measurements captures the temporal variability and improves the ADP/solution rms difference. Despite significant temporal variability, persistent features are found for the studied period; for instance, the dominant direction of baroclinic wave phase and energy propagation is always from the northwest. At the surface, baroclinic surface tidal currents (deviations from the depth-averaged current) can be 10 cm s21, 2 times as large as the depth-averaged current. Barotropic-to-baroclinic energy conversion is generally weak within the model domain over the shelf but reaches 5 mW m22 at times over the slopes of Stonewall Bank.

The Generation and Propagation of Sea Level Variability Along the Pacific Coast of Mexico

Abstract Case history analysis, cross spectra and multiple regression analysis have been used in a study of low-pass filtered sea level records from the Pacific mainland coast of Mexico in 1971 and 1973–75. During the summer-fall season (May–October), sea level variability is characterized by strong alongshore coherence and nondispersive, poleward phase propagation over a wide frequency range (0.02–0.37 cpd). The strength and clarity of the propagating signals seem to be related primarily to large-amplitude events of elevation (10–30 cm) that are generated off the southern coast of Mexico by tropical storms. These events are typically forced by the alongshore, poleward movements of the storms to as far north as 20°N, and thereafter continue to propagate freely at least as far as Guyamas (28°N). Large, variable phase speeds (250–500 km day−1 are observed in the southern region, consistent with the alongshore speeds of the forcing. A multiple-input statistical forcing model, in which adjusted sea level is r...