N. Perenne et al., RECTIFIED BAROTROPIC FLOW OVER A SUBMARINE-CANYON, Journal of physical oceanography, 27(9), 1997, pp. 1868-1893
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Categorie Soggetti
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1868 - 1893
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The effect of an isolated canyon interrupting a long continental shelf of constant cross section on the along-isobath, oscillatory motion of a homogeneous, incompressible fluid is considered by employing labora tory experiments (physical models) and a numerical model. The laborato ry experiments are conducted in two separate cylindrical test cells of 13.0- and 1.8-m diameters, respectively. In both experiments the shel f topography is constructed around the periphery of the test cells, an d the oscillatory motion is realized by modulating the rotation rate o f the turntables. The numerical model employs a long shelf in a rectan gular Cartesian geometry. It is found from the physical experiments th at the oscillatory how drives two characteristic how patterns dependin g on the values of the temporal Rossby number, Ro(i), and the Rossby n umber, Ro. For sufficiently small Ro(i), and for the range of Ro inves tigated, cyclonic vortices are formed during the right to left portion of the oscillatory cycle, facing toward the deep water, on (i) the in side right and (ii) the outside left of the canyon; that is, the cyclo ne regime. For sufficiently large Ro(i) and the range of Ro studied, n o closed cyclonic eddy structures are formed, a flow type designated a s cyclone free. The asymmetric nature of the right to left and left to right phases of the oscillatory, background flow leads to the generat ion of a mean how along the canyon walls, which exits the canyon regio n on the right, facing toward the deep water, and then continues along the shelf break before decaying downstream. A parametric study of the physical and numerical model experiments is conducted by plotting the normalized maximum mean velocity observed one canyon width downstream of the canyon axis against the normalized excursion amplitude X. Thes e data show good agreement between the physical experiments and the nu merical model. For X greater than or equal to 0.4, the normalized, max imum, mean velocity is independent of X and is roughly equal to 0.6; i .e., the maximum mean velocity is approximately equal to the mean forc ing velocity over one half of the oscillatory cycle (these experiments are all of the cyclone how type). For X less than or equal to 0.4, th e normalized maximum mean velocity separates into (i) a lower branch f or which the mean flow is relatively small and increases with X (cyclo ne-free flow type) and (ii) an upper branch for which the mean flow is relatively large and decreases with X (cyclone how type). The time-de pendent nature of the large-scale eddy field for a numerical model run in the cyclone regime is shown to agree well qualitatively with physi cal experiments in the same regime. Time-mean velocity and streamfunct ion fields obtained from the numerical model are also shown to agree w ell with the laboratory experiments. Comparisons are also made between the present model findings and some oceanic observations and findings From other models.