Open AccessConvergence fronts in tidally forced rotating estuaries
Author(s) -
Handler Robert A.,
Mied Richard P.,
Evans Thomas E.,
Donato Timothy F.
Publication year - 2001
Publication title -
journal of geophysical research: oceans
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.67
H-Index - 298
eISSN - 2156-2202
pISSN - 0148-0227
DOI - 10.1029/2000jc000637
Subject(s) - bathymetry , geology , shoal , vorticity , estuary , flow (mathematics) , eddy , current (fluid) , geodesy , mechanics , oceanography , vortex , physics , turbulence
In situ observation and remote sensing imagery reveal the presence of longitudinal velocity convergences over bathymetric channels in tidal estuaries. We present the results of numerical experiments designed to investigate the cause of these convergences for channels possessing shallow shoal regions and a deeper central region. The equations of motion for a homogeneous fluid 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 variations (in the y direction) are permitted. A Gaussian‐shaped bottom bathymetry is chosen. In the along‐channel ( y ) direction we impose a pressure gradient which is the sum of constant and fluctuating parts to simulate the steady and tidally oscillating parts of the estuarine flow. The details of the transient response can be complicated, but we observe that for most (∼80%) of the tidal cycle there exists a cross‐estuary recirculation cell colocated with a localized along‐channel jet. Both of these are situated over the bottom bathymetric groove; the circulation is always clockwise when 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 associated 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 center as long as the current continues in the same direction. When the tide reverses, 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 asymmetrically 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 Ekman layer thickness‐to‐channel‐depth ratio is greater than ∼0.65.