The Subtle Effects of Topography

Abstract

Constant depth models of shallow seas discussed in the previous two chapters have elucidated many interesting dynamical phenomena in particularly simple ways, but of course these models are overidealized. Before attempting any comparison between the theoretical models and observation, the dynamical effects of variable depth on basin response have to be investigated. All natural basins possess a complex depth distribution, characterized in particular by a depth reducing gradually to zero at the shores. Figure 4.1 shows the depth distribution of Lake Ontario by a contour map; a cross section near the western end is shown later (Figure 4.5). It should be remembered that the vertical scale on such cross sections is exaggerated by a large factor (200 in Figure 4.5): without such an exaggeration the depth distribution would simply appear as a thickened horizontal line. Figure 4.2 shows the depth distribution off the south coast of Long Island. The near-shore parts of Figurew 4.5 and 4.2 are generally similar and show bottom slopes of the order of 10⁻². The flatness of the bottom of a typical coastal ocean basin would lead one to think that constant depth models should be reasonably realistic, except very close to shore where the relative changes in depth are rapid (change in depth relative to local depth). In a way this is true and the phenomena revealed by an analysis of constant depth models have their observable counterparts in real basins. However, there are also other modes of motion which only occur in basins of variable depth. Furthermore, some phenomena, present over whatever depth distribution, have their character qualitatively modified by depth variations, sometimes in quite subtle and unexpected ways. Particularly interesting effects arise from an interaction of depth variations and the Earth’s rotation.