Vortex pairs formed by tidal currents in the Naruto Strait: effects of bottom topography, density stratification, and coastline geometry

Abstract

Vortex pairs formed at a strait outlet by tidal flow effectively induce water exchange and material transport in coastal areas. However, the effects of complex bottom topography and density stratification remain unclear. Here, we investigated the development and propagation of vortex pairs in the Naruto Strait, which has complex topography. Satellite observations indicated that the vortex pairs formed on the northern side of the strait continue to move away from the strait after the reversal of tidal flow, shifting their propagation from northward to westward. Numerical experiments revealed that: (1) changes in depth affect the propagation speed and overall size of vortex pairs; (2) density stratification reduces the effects of depth changes; (3) coastline geometry affects the propagation direction of vortex pairs. Furthermore, experiments with idealized topography showed that in a region where depth increases with vortex-pair propagation, the jet decelerates, and the vortex pair shrinks in size. Conversely, in a region where depth decreases, the jet widens, and the vortex pair expands. The changes in jet flow speed can be attributed to flow continuity and depth change, as the latter alters the cross-sectional area. Meanwhile, the changes in vortex pair size and jet width can be explained by vortex propagation on a slope due to potential vorticity conservation. These effects of topography and density stratification may also be significant in other coastal areas and potentially influence the Strouhal number threshold below which vortex pairs leave an outlet.