Longitudinal convergence fronts in homogeneous rotating channels

In situ observation and remote sensing imagery indicate the presence of vel ocity convergences located over bathymetric channels in the mouths of tidal estuaries. In this paper we present the results of numerical simulations p erformed to investigate these velocity structures in a rotating channel hav ing a single bathymetric groove. The equations of motion for a homogeneous fluid on a rotating Earth are solved using a fully spectral code in the acr oss-channel (i.e., the vertical or x-z) plane. No along-channel flow variat ions (in the y direction) are permitted. The bottom bathymetry is formed us ing a unique virtual surface approach [Goldstein et al., 1993] that generat es a no-slip bottom using feedback forcing. A Gaussian-shaped channel is em ployed to simulate typical estuarine bathymetry. In the along-channel direc tion a constant pressure gradient is imposed, and the flow evolves until a steady state results. The simulations are performed at high Rossby number ( of order unity) based on the width of the groove and a typical surface velo city. Simulations show the development of a localized along-channel jet col ocated with an across-channel recirculation cell. This feature results from the generation of streamwise vorticity through the tilting of planetary vo rticity by the vertical shear in the along-channel flow. The associated acr oss-channel surface flow above the jet exhibits convergent and divergent re gions, which correlate reasonably well with features reported previously in the literature. Their number, position, and strength are seen to vary with the along-channel Reynolds number, Ekman layer thickness, and channel aspe ct ratio.