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  • Author or Editor: D. B. Haidvogel x
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D. B. Haidvogel

Abstract

Numerical simulations are used to study on-shelf transport of dense water by oscillatory barotropic currents incident upon an isolated coastal canyon. The physical system is a laboratory-scale annulus in which forcing is provided by an oscillatory modulation in the rotation rate and for which the external nondimensional parameters match those from an appropriate oceanic analog. The numerical simulations were conducted for comparison with a companion set of laboratory experiments, and these prior studies have shown good qualitative and quantitative agreement between the two. Here the numerical simulations are examined in further detail to determine the three-dimensional structure of the time-mean currents and density field on the shelf, to quantify the resulting onshore transport of dense water, and to expose the underlying nondimensional parameter dependencies. The incident barotropic currents produce, in equilibrium, an extensive pool of anomalously dense fluid on the shelf surrounding the canyon. This dense pool is maintained on the shelf by a combination of cross-shelf mean and eddy density fluxes. The associated time-mean pools of dense water on the shelf have considerable spatial structure, most prominently a vertically thick lens of anomalously dense water along the upstream limb of the canyon, accompanied by strong anticyclonic residual currents. The dense pool occupies an area and volume that are large when compared with that of the canyon itself. Several bulk measures of on-shelf exchange of dense water are devised to facilitate comparison across the simulations performed. This comparison of bulk measures indicates that net on-shelf transport of dense water varies linearly with forcing period (inverse temporal Rossby number) and quadratically with forcing strength (Rossby number). Variations in on-shelf pumping with Burger number are found to be weak. A simple dynamical scaling argument is consistent with the observed dependencies.

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D. B. Haidvogel and G. Holloway
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B. L. Hua and D. B. Haidvogel

Abstract

Direct simulations of turbulent quasi-geostrophic flow on a βplane have been performed with an emphasis on three-dimensional resolution [128 × 128] in the horizontal and up to 6 vertical modes). The statistically equilibrated turbulence is forced by baroclinic instability of a mean shear, and dissipation occurs through friction in a bottom Ekman layer. Parameters which control the vertical structure of the variability and the energy partition between the vertical modes are identified. We verity Charney's prediction of three-dimensional isotropization of quasi-geostrophic flows in the case of a constant Brunt-Väisälä profile; for a variable Brunt- Väisäl&auml stratification spectral shapes are preserved but isotropization is lost. All baroclinic modes show a tendency for a direct energy cascade, as opposed to the red cascade of purely barotropic flow. The coalescence of potential vorticity into three-dimensionally isolated structures is observed to occur; however, their vertical scale is highly dependent on whether they appear in a forced turbulent flow or in freely evolving turbulence, the latter having a larger vertical scale.

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A. R. Robinson and D. B. Haidvogel

Abstract

The initial/boundary value problem for the barotropic version of a quasi-geostrophic open ocean model which requires normal flow everywhere on the boundary and vorticity on the inflow is studied. Parameter dependencies and sensitivities are determined for dynamical forecast experiments carried out over a 500 square kilometer domain with data simulated to represent the mid-ocean eddy field at 1500 m. The computational rms forecast error due to open boundary conditions is kept to 5% after one year of integration. Errors, gaps and noise are then introduced into the boundary and initial condition data. Objective analysis is introduced for mapping coarsely-distributed data onto the computational grid, and vorticity is derived from the streamfunction by several methods. Forecast error is sensitive to the frequency of updating of boundary data, but generally insensitive to vorticity errors. A simulated forecast experiment with composite error sources representative of feasible oceanic conditions is carried out for four months duration with rms error maintained to about 10%.

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D. B. Haidvogel and K. H. Brink

Abstract

A sequence of numerical simulations is described of wind-driven flow over irregular continental shelf topography. The model is barotropic, nonlinear, and forced by a periodic, spatially uniform alongshelf wind stress. The objective of the study is to determine whether topographic drag, known to be asymmetric for barotropic flow over the shelf, can generate substantial time-averaged alongshore currents in the presence of a fluctuating zero-mean wind stress.

With realistic parameters, mean maximum alongshore currents of 0.05 to 7.0 cm s−1 are realized with flow in the direction of freely propagating shelf waves. The residual current strength is a strong function of wind stress period and bottom bump wavelength: larger forcing periods and shorter bump wavelengths enhance the time-mean circulation. Particle paths are generally observed to be chaotic, in contrast to the nearly cyclic behavior of the Eulerian velocity field. However, cross-shore particle dispersion is well correlated with the mean alongshore currents and may represent a testable observational signature of topographic drag effects.

Model simulations using realistic spectra for both wind stress and bottom roughness yield a maximum flow of approximately 2.5 cm s−1. These results demonstrate that topographic drag asymmetries can lead to observable mean currents on continental shelves and may be a partial explanation for certain observed mean currents that run counter to mean alongshore winds.

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John D. McCalpin and Dale B. Haidvogel

Abstract

The low-frequency variability of the oceanic wind-driven circulation is investigated by use of a reduced-gravity, quasigeostrophic model with slight variations on the classic double-gyre wind forcing. Approximately 30 eddy-resolving simulations of 100–1000 years duration are analyzed to determine the types of low-frequency variability and to estimate statistical uncertainties in the results.

For parameters close to those leading to a stable antisymmetric solution, the system appears to have several preferred phenomenological regimes, each with distinct total energy levels. These states include a high-energy quasi-stable state; a low-energy, weakly penetrating state; and a state of intermediate energy and modest eddy/ring generation. The low-frequency variability of the model is strongly linked to the irregular transitions between these dynamical regimes.

For a central set of reference parameters, the behavior of the system is investigated for each period in which the total energy remains in certain ranges. The structure of the time-averaged streamfunction and eddy energy fields are observed to have remarkable repeatability from event to event for each state.

A parameter study documents the ways in which the probability distribution function of the total energy depends on the strength and asymmetry of the wind forcing field. As the parameters shift away from those leading to a steady antisymmetric solution, we find that increasing the asymmetry of the wind field or reducing the viscosity decreases the occurrences of the high-energy, quasi-stable state. The low-energy, weakly penetrating state is more robust and exists whenever there is both instability and a certain minimal asymmetry in the forcing. As the wind asymmetry is increased, the distributions shift smoothly (but rapidly) away from the higher-energy states, until only the low-energy state remains.

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J. Adams, R. Garcia, B. Gross, J. Hack, D. Haidvogel, and V. Pizzo

Abstract

Elliptic partial differential equations from different areas in the atmospheric sciences are easily and efficiently solved using the multigrid software package mudpack. It is shown that the multigrid method is more efficient than other commonly used methods, such as Gaussian elimination and fixed-grid relaxation. The efficiency relative to other methods, both in terms of computational time and storage requirement, increases rapidly with grid size.

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J. C. Muccino, H. Luo, H. G. Arango, D. Haidvogel, J. C. Levin, A. F. Bennett, B. S. Chua, G. D. Egbert, B. D. Cornuelle, A. J. Miller, E. Di Lorenzo, A. M. Moore, and E. D. Zaron

Abstract

The Inverse Ocean Modeling (IOM) System is a modular system for constructing and running weak-constraint four-dimensional variational data assimilation (W4DVAR) for any linear or nonlinear functionally smooth dynamical model and observing array. The IOM has been applied to four ocean models with widely varying characteristics. The Primitive Equations Z-coordinate-Harmonic Analysis of Tides (PEZ-HAT) and the Regional Ocean Modeling System (ROMS) are three-dimensional, primitive equations models while the Advanced Circulation model in 2D (ADCIRC-2D) and Spectral Element Ocean Model in 2D (SEOM-2D) are shallow-water models belonging to the general finite-element family. These models, in conjunction with the IOM, have been used to investigate a wide variety of scientific phenomena including tidal, mesoscale, and wind-driven circulation. In all cases, the assimilation of data using the IOM provides a better estimate of the ocean state than the model alone.

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