Convergence fronts in tidally forced rotating estuaries

In situ observation and remote sensing imagery reveal the presence of longi tudinal velocity convergences over bathymetric channels in tidal estuaries. We present the results of numerical experiments designed to investigate th e cause of these convergences for channels possessing shallow shoal regions and a deeper central region. The equations of motion for a homogeneous flu id on a rotating Earth are solved using a fully spectral code in the across -estuary (i.e., the vertical or x-z) plane, while no along-estuary flow var iations (in the y direction) are permitted. A Gaussian-shaped bottom bathym etry is chosen. In the along-channel (y) direction we impose a pressure gra dient which is the sum of constant and fluctuating parts to simulate the st eady and tidally oscillating parts of the estuarine flow. The details of th e transient response can be complicated, but we observe that for most (simi lar to 80%) of the tidal cycle there exists a cross-estuary recirculation c ell colocated with a localized along-channel jet. Both of these are situate d over the bottom bathymetric groove; the circulation is always clockwise w hen facing down current. This feature results from the generation of stream -wise vorticity through the tilting of planetary vorticity by the vertical shear of the along-estuary flow. A surface convergence-divergence pair is a ssociated with the flow. The maximum value of each is seen to occur on the edge of the bathymetric feature but may migrate toward or away from the cen ter as long as the current continues in the same direction. When the tide r everses, the feature reappears on the opposite shoal, and the migration of the convergence and divergence extrema begins again. We also find that the responses are qualitatively similar for all bathymetric grooves, even asymm etrically situated ones, provided that the estuary width-to-depth ratio is of order 100 or larger, the Rossby numbers are of order unity, and the Ekma n layer thickness-to-channel-depth ratio is greater than similar to0.65.