Convergence fronts in tidally forced rotating estuaries

Abstract

Abstract. 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 alongestuary 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 crossestuary 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.

River velocities from sequential multispectral remote sensing images

“…However, the southern shore between Metomkin and Mathias Points exhibits the presence of an elongated clockwise recirculation (denoted with a red arrow), which may have two possible sources. It may have been shed by the accelerating ebb flow around the triangular Metomkin cape [ Black and Gay , ; Signell and Geyer , ], or it may be driven by the difference in the enhanced influence of bottom friction in the shallow water compared to that in the deeper fluid as the tide changes from flood to ebb [ Handler et al ., ]. A smaller, but perhaps less well understood, anticlockwise eddy can be seen north and slightly west of Metomkin Pt on the northern shore (red arrow).…”

“…This is because damping of the flow occurs principally through bottom friction, so that a shallow flow is more rapidly decelerated than a deep one, since the shear stress at the bottom is communicated over a smaller vertical distance. The result is that when the ebb/flood changeover occurs, the shallow flow can have slowed to zero and reversed direction before the bottom friction has a chance to appreciably decelerate the thalweg motion [ Handler et al ., ].…”

Frontogenesis and Frontal Progression of a Trapping-Generated Estuarine Convergence Front and Its Influence on Mixing and Stratification

“…For further details on RiverRad operation during this experiment, see Plant et al (2009). As has been shown in other radar applications to fronts (e.g., Braun et al 2008;Handler et al 2001), radar backscatter peaks near the frontal location.…”

Ocean processes underlying surface clustering

“…Such features are regularly observed from satellites, aircraft, and ships [ Gower et al ., ; Munk et al ., ; Yoder et al ., ]. Analogous structures are also observed in estuaries and rivers [ Handler et al ., ; Mied et al ., ]. At the largest scales, clusters include such phenomena as the “great garbage patches” in the ocean gyres driven by convergent surface Ekman currents [ Maximenko et al ., ].…”