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Francisco E. Werner and Barbara M. Hickey

Abstract

In this paper we demonstrate the importance of the seasonal barotropic longshore pressure gradient force to Pacific Northwest coastal dynamics. Values of the seasonal longshore pressure gradient corrected for gauge height relative to a level surface (Hickey and Pola, 1982) and for year-to-year variations (Enfield and Alien, 1980) were included in the two-dimensional, time-dependent, baroclinic finite-difference model of Hamilton and Rattray (1978) as an external force. Observed wind stress, stratification and bottom topography were included in the model, and comparisons were made with current meter data in each of the three seasonal situations observed in the Northwest: pressure gradient force southward opposing local wind stress (winter), pressure gradient force northward opposing local wind stress (summer), and pressure gradient force and local wind stress both southward (spring). Three important features of the seasonal circulation are shown to depend on the existence of the pressure gradient force: the northward undercurrent (the California Undercurrent) that exists along most of the West Coast during summer, a southward undercurrent (which we denote the Washington Undercurrent) that the model predicts and observations shown herein substantiate for the Pacific Northwest slope during winter, and the anomalously strong southward flow that occurs subsequent to the spring transition. Finally, the seasonal variation of the balance between wind stress, bottom stress and vertically integrated longshore pressure gradient force as a function of bottom depth is addressed. In particular, it is shown that bottom stress is significant in the mid-shelf region during both winter and spring. The commonly made assumption that offshore transport in the surface layer is balanced by onshore transport in the inviscid interior is shown to be invalid except during summer.

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Robert S. Webb and Francisco E. Werner
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Thomas F. Gross and Francisco E. Werner

Abstract

Tidal flows over irregular bathymetry are known to produce residual circulation flows due to nonlinear interaction with gradients of depth. Using the depth-averaged vorticity equations, the generation of residual vorticity and residual flows due to variation of the frictional coefficient are examined. The authors find that the contribution due to bottom roughness variations can be as large as that arising from gradients of depth and velocity. Specific cases are considered on the northern California shelf, Georges Bank, and the U.S. South Atlantic Bight.

The generation of residual vorticity is a strong function of the length scales at which roughness or depth vary. Length scales of bottom roughness variation are commonly within the range of greatest effect (e.g., sand patchiness, cobbly outcrops, etc.). The site-specific cases show that the bottom roughness variability can generate as much residual circulation as that expected from depth variability. The implication for numerical modeling studies is that resolution of roughness variability is as important as resolution of topography at length scales comparable to the tidal excursion. Therefore higher-resolution models that seek to resolve flow patterns due to tidal scale topographic variability will also require similarly resolved bottom roughness variability.

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Francisco E. Werner, Alán Cantos-Figuerola, and Gregorio Parrilla

Abstract

The reduced-gravity response of an hydraulically-forced flow in a rectangular channel is examined numerically. The model parameters were set by observed features of the circulation in the westernmost basin of the Mediterranean Sea—the Alborán Sea. Numerical experiments reveal that the solution depends critically on the inclusion of the momentum advection terms and on the form of the boundary condition on the domain's sidewalls. In particular, while free-slip conditions show a coastally trapped current on the model's southern boundary, no-slip conditions cause the formation of an anticyclonic gyre upon the flow's entry into the basin.

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David A. Greenberg, Francisco E. Werner, and Daniel R. Lynch

Abstract

A spherical-polar 3D, harmonic-in-time, linear finite-element ocean circulation model that allows for spatially arbitrary forcing by winds, density field, and imposed free-surface elevation along open boundaries is presented. Model features and capabilities are demonstrated in applications to idealized and realistic basin-scale flows. Subpolar, subtropical, and equatorial flows are reproduced with imposed wind fields in models with simplified geometry. A North Atlantic model captures features of the baroclinic circulation including the Gulf Stream and deep western boundary currents. The flow along the northwest Atlantic continental shelf is used to demonstrate boundary forcing and boundary outflow conditions. The run time of the model solutions makes this a desirable tool for 3D diagnostic simulations of large datasets.

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Michael G. Jacox, Michael A. Alexander, Nathan J. Mantua, James D. Scott, Gaelle Hervieux, Robert S. Webb, and Francisco E. Werner
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