Showing papers by "Kenneth H. Brink published in 1987"

A stochastic model for wind-driven currents over the continental shelf

Abstract: A stochastic model for current, pressure, and density fluctuations over the continental shelf and slope is solved for coherence, phase, and gain between the oceanographic variables arid wind stress. Comparison of wind stress spectra and transfer functions show that free coastal-trapped wave physics tends to exist in a frequency band bounded at the low end because of frictional predominance and at the high end because of the absence of wind energy at appropriate alongshore length scales. The transfer functions for density and especially cross-shelf velocity show that these variables are sensitive to short length scales in the forcing and are thus difficult to predict in general. Model results are compared to observations of bottom pressure and alongshore velocity from the 1982 Coastal Ocean Dynamics Experiment off northern California. The results agree with observations, at least qualitatively, with regard to spatial and frequency patterns in coherence and gain. The poorest point of agreement is in the amplitude of wind stress-current gains, which the model systematically underpredicts. The comparisons do demonstrate the importance of including accurate representations of bottom friction and of both cross-shelf and alongshore gradients of wind stress amplitude.

Shelf and slope circulation induced by fluctuating offshore forcing

Abstract: A linear model which includes continuous vertical stratification, arbitrary cross-shelf bottom topography, and bottom friction is used to examine the response of shelf and slope waters to fluctuating offshore forcing in the form of a specified pressure field. For forcing which is periodic in the alongshore direction and in time, the response varies dramatically with frequency. For periods of less than about 10 days, the response is dominated by near resonances with free coastally trapped waves. These are not pure resonances because of the mismatch between the forcing structure and the free wave structure. For periods of greater than about 10 days, the velocity response decays away from the forcing with a scale determined by the projection of the forcing onto the flat-bottom baroclinic modes, typically the first baroclinic Rossby radius. If the continental slope is encountered within this scale distance, then the flow is altered and creates a bottom-trapped, alongshore velocity maximum seaward of the shelf break. Increased stratification enhances bottom trapping and inhibits flow across the slope, moving the maximum in alongshore velocity seaward, thus reducing the alongshore flow near the shelf break. The shelf response is always weak and barotropic. Furthermore, the slope response is largely independent of the shelf geometry, suggesting that narrow shelves appear to be more easily influenced by offshore forcing only because their coasts are closer to the forcing. The model is used to simulate the response to a Gaussian-shaped anticyclonic eddy translating uniformly in the alongshore direction. The eddy flow is blocked by the topography and becomes squashed at the shelf break. Some shelf water is entrained creating a weak shelf circulation cell. The shoreward flow in the eddy cannot move onto the shelf and instead forms an alongshore jet near the shelf break. The jet extends away from the eddy in the direction toward which free coastally trapped waves propagate, but the alongshore velocity within the jet is opposite to the alongshore velocity within the eddy.

Coastal ocean physical processes

Abstract: In the most recent paper of this series, Allen et al. [1983] used a geographical approach to summarize United States progress in continental shelf physical oceanography. Their approach tended to emphasize observational results and differences among regions. This installment in the series will take an alternative tack, by trying to summarize progress in terms of understanding particular physical processes. This approach cannot escape being at least somewhat geographical, however, because different processes tend to predominate in different parts of the world.

Upwelling fronts: implications and unknowns

Abstract: Existing observations of coastal upwelling fronts and of upwelling filaments are reviewed Attention is given to isolating those aspects where dynamic understanding and knowledge of physical-biological coupling are weak Theoretical results are then reviewed with the goal of providing some insight on those issues In view of shortfalls in present knowledge, two interdisciplinary field studies are suggested to examine the stability of upwelling fronts and the mechanisms of exchange across them