Subtidal secondary circulation induced by eddy viscosity-velocity shear covariance in a predominantly well-mixed tidal inlet (original) (raw)
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Transient Tidal Circulation and Momentum Balances at a Shallow Inlet
Journal of Physical Oceanography, 2003
An analysis of transient momentum balances is carried out to elucidate circulation, dynamics, and exchange mechanisms at shallow barotropic tidal inlets. Circulation is computed using a depth-integrated, fully nonlinear, time-stepping, finite-element model with variably spaced grids having horizontal resolution down to 50 m. Velocity and elevation fields from the model are used to directly evaluate the contribution of each term in the momentum equations to the overall momentum balance. A transformation of the x-y momentum terms into an s-n coordinate system is used to simplify the interpretation of the dynamics and provide vivid illustrations of the forces and resulting accelerations in the flow. The analysis is conducted for an idealized inlet and contrasted with a highly detailed model of Beaufort Inlet, North Carolina. Results show that momentum balances in the immediate vicinity of these inlets vary significantly in time and space and oscillate between two dynamical states. Near maximum ebb or flood, the alongstream momentum balances are dominated by advective acceleration, pressure gradient, and bottom friction. Cross-stream balances are dominated by centrifugal acceleration and pressure gradients. Near slack, balances more closely follow linear wave dynamics, with local accelerations balancing pressure gradients, and (to a lesser degree) Coriolis. Comparisons between the idealized inlet and Beaufort Inlet show broad similarities in these momentum balances. However, natural inlet geometry and bottom topography, as well as the tidal transmission characteristics of the sounds behind Beaufort Inlet produce strong asymmetries. Moreover, momentum balances are highly localized, often with subkilometer length scales. The dynamics are used to explain the physical mechanisms for inlet exchange. In particular, the results indicate that the cross-stream dynamics generate a ''wall'' along the length of an inlet during the stronger phases of the tide. The wall is established by opposing cross-inlet pressure gradients and centrifugal forces, and it poses a significant barrier to cross-inlet exchange during the stronger phases of the tide but is absent near slack.
Vertical circulation in shallow tidal inlets and back-barrier basins
Continental Shelf Research, 2007
In this paper, we analyse the contribution of tidally induced drift in the surface layer to the overall dynamics of wellmixed tidal basins undergoing drying and flooding. The study area covers the East Frisian Wadden Sea (German Bight, Southern North Sea), which consists of seven tidal basins. The major interest is focused on the tidal basin behind the islands of Langeoog and Spiekeroog and the inlet connecting it with the North Sea. The comparison between theoretical concepts, results from direct observations, and simulations with a numerical model helps to understand the underlying physics controlling the tidal response. The data were collected during the period 1995-1998 and consist of cross-channel ADCP transects. The identification of the dominant spatial patterns and their temporal variability is facilitated by applying an EOF analysis to the data. The numerical simulations are based on the 3-D primitive equation General Estuarine Transport Model (GETM) with a horizontal resolution of 200 m and terrain-following vertical coordinates. We find distinct differences between the temporal variability of the transports near the surface and those in deeper layers of the tidal inlets. The near surface transport is dominated by the tidally induced drift (similar to the Stokes drift), whereas the deeper layer transport is dominated by asymmetries caused by the hypsometric properties of the intertidal basins. These transports, when averaged over a tidal period, have opposite directions and compensate each other. This explains the establishment of a vertical overturning cell: landward motion in the upper layers and seaward motion in the deeper parts of the tidal channels. This vertical circulation cell is also observable in our numerical simulations and shows a clear dependency of the temporal asymmetry in the transport patterns on the local depth. In deep tidal channels, the overall properties of the tidal signal show a clear ebb dominance, whereas in the shallow extensions of the channels the transports during flood are larger than during ebb. Although, our research area can be characterized as a well mixed estuary, baroclinicity associated with the fresh water flux from the coast can substantially affect vertical overturning. r
Lateral Circulation in a Partially Stratified Tidal Inlet
2018
Using a three-dimensional, hydrostatic, primitive-equation ocean model, this study investigates the dynamics of lateral circulation in a partially stratified tidal inlet, the Barataria Pass in the Gulf of Mexico, over a 25.6-hour diurnal tidal cycle. Model performance is assessed against observational data. During flood tide, the lateral circulation exhibits the characteristics similar to that induced by differential advection, i.e., lateral flow consists of two counter-rotating cells and is convergent at the surface. The analysis of momentum balance indicates that, in addition to the pressure gradient and vertical stress divergence, nonlinear advection and horizontal stress divergence are also important contributors. During ebb phase, the lateral circulation is mostly eastward for the whole water column and persisting for almost the whole period. The surface divergence suggested by the differential advection mechanism lasts for a very short period, if it ever exists. The main momen...
Role of asymmetric tidal mixing in the subtidal dynamics of narrow estuaries
Journal of Geophysical Research: Oceans, 2013
The role of asymmetric tidal mixing (ATM) in subtidal estuarine dynamics is investigated using a series of generic numerical experiments that simulate narrow estuaries under different stratification and external forcing conditions. The focus is on quantifying the characteristics of ATM-induced flow and its contributions to stratification and salt transport. The flow induced by ATM has a two-layer vertical structure in periodically stratified estuaries, similar to that of the density-driven flow. It has a three-layer vertical structure in the central regime of weakly stratified estuaries, and a reverse two-layer structure in highly stratified estuaries. The changes in vertical distribution of ATM-induced flows result from the influence of stratification on the covariance of eddy viscosity and vertical shear. Such covariance represents the driving force of ATM-induced flow in the tidally averaged momentum equation. Compared to density-driven flow, ATM-induced flow dominates in periodically stratified estuaries with strong tides, has the same order of magnitude in weakly stratified estuaries with moderate tides, and is less important in highly stratified estuaries with weak tides. In contrast to density-driven flow that always increases estuarine stratification and transports salt landward, the ATM-induced flow exhibits different behaviors because of its varying vertical structure. In estuaries with strong tides, ATMinduced flow tends to enhance stratification and to transport salt landward, similar to density-driven flow. In estuaries with weak tides, ATM-induced flow tends to reduce stratification and to transport salt seaward.
Circulation, mixing, and exchange processes in the vicinity of tidal inlets: A numerical study
Journal of Geophysical Research: Oceans, 1996
The circulation in the vicinity of an idealized tidal inlet connecting a continental shelf and a coastal sound is examined. The circulation is forced by an M2 tide and a weakly buoyant discharge. The buoyant discharge forms a plume in the coastal ocean and induces a distinct anticyclonic circulation at the plume edge that is maintained throughout the tidal cycle. We focus on the plume's onset and its evolution over 5–10 tidal cycles. Over the timescales considered, the plume was roughly circular, slightly skewed in the along‐coast direction. The model solution yielded high vertical Ekman numbers Eϵ ∼ O(5) in the vicinity of the inlet mouth, decreasing seaward from the inlet to an order of magnitude smaller (Eϵ ∼ 0.25) at the seaward edge of the plume. Passive particles released in the region seaward of the inlet mouth were used to describe the exchange between the coastal region and the inlet. A marked asymmetry between ebb and flood flows is observed in the vicinity of the inle...
Lateral dynamic analysis and classification of barotropic tidal inlets
Continental Shelf Research, 2002
The dynamical balances at shallow tidal inlets are highly nonlinear, and can vary substantially over sub-kilometer scales. In this study, barotropic dynamics are examined with numerical experiments on a series of idealized inlets with differing inlet widths and lengths. Circulation and elevation fields obtained from fully nonlinear depth-integrated circulation models are used to reconstruct the contribution of each term in the momentum equations. Momentum terms are rotated into a streamline coordinate system to simplify interpretation of the dynamics. Spatial patterns in momentum reveal that the lateral balances at inlets can vary from nearly geostrophic to strongly cyclostrophic. Marked dynamical differences are seen between inlets with different lengths and widths. Inlet regions of geostrophic or cyclostrophic balances can be predicted using two dimensionless parameters, the dynamic length L Ã and dynamic width W Ã : A classification scheme is proposed using L Ã and W Ã to compare the idealized inlets analyzed here with inlets from 20 previous studies. Four distinct inlet types are identified and discussed.
Journal of Geophysical Research, 2011
Residual currents induced by asymmetric tidal mixing (ATM) were examined using a series of idealized numerical experiments for weakly stratified, partially mixed, and highly stratified narrow estuaries that neglected lateral variations of bathymetry and the effects of Earth's rotation. The Eulerian residual currents were decomposed into four components, i.e., river-induced, density-driven, nonlinearities-induced, and ATM-induced flows such that the longitudinal distribution and strength of each component can be depicted and evaluated. In weakly stratified estuaries, ATM-induced flow has a two-layer structure similar to that of density-driven flow. It reinforces the estuarine exchange flow. In partially mixed and highly stratified estuaries, the ATM-induced flow tends to have a three-layer structure with landward flows near the surface and the bottom and seaward flow in the middle of the water column. It appears to act against the estuarine exchange flow in parts of the water column. The relative importance of ATM-induced flow to estuarine residual currents varies in different types of estuaries. Compared to the density-driven flow, the relative importance of ATM-induced flow decreases as stratification increases. In the central regime of the estuary, the strength of the ATM-induced flow is greater than that of density-driven flow under weak stratification and tends to be smaller under strong stratification.
Tidal-induced buoyancy flux and mean transverse circulation
Continental Shelf Research, 1986
A weakly nonlinear model is used to examine the mean transverse circulation (crossisobath) driven by tidal-induced buoyancy flux. The mean Eulcrian flows driven by both the barotropic and baroclinic tide are presented for a semi-infinite wedge. The mean flow driven by the barotropic tide is significant only near the apex where the thickness of the frictional boundary layer is comparable to the water depth. The mean flow there is characterized by a single-cell circulation with offshore flow near the bottom, and its magnitude can reach a few percentage or a significant fraction of the tidal velocity in oceanic applications. The mean flow driven by the baroclinic tide, on the other hand, is characterized by pairs of half-open (on the seaward side) counter-rotating cells, the number of which equals the vertical mode number. For a baroclinic tide propagating onshore, the mean flow near the top and bottom surfaces is always directed offshore and its magnitude can reach a large fraction of the tidal velocity. Taken together, the model thus predicts a mean offshore flow near the bottom while higher up in the water column the mean flow direction is less definite due to the contribution from different tidal components. The model results are consistent with some current measurements over the Georges Bank.
Ocean Modelling, 2020
In this paper, the hydrodynamics of the Southern Bight of the North Sea (SBNS) and in particular, the Belgian Coastal Zone (BCZ) is investigated on daily to seasonal time scales using a high resolution hydrodynamical model. The Regional Ocean Modeling System (ROMS) is implemented over the SBNS with 5 km resolution and downscaled at 1 km resolution over the BCZ in a two-way nesting configuration run over a three years period (i.e. 2006-2008). The benefit of using a high resolution model over the BCZ is assessed through an extensive comparison of model results with data from satellite and in-situ fixed platforms as well as reference products available for the region. The validation exercise and the results analysis are conducted with a particular focus on hydrodynamic features that are expected to impact the sediment transport. We find that despite the validation procedure does not allow to clearly demonstrate better performance of the high resolution model compared to the coarse resolution model in terms of overtidal circulation, sea surface temperature (SST) and salinity (SSS), the high resolution model resolves additional details in the variability of residual circulation and Scheldt salinity plume dynamics. The analysis of the response of the simulated hydrodynamics to atmospheric regimes for neap and spring tide highlights the major role played by the wind direction on the averaged currents and plume extension. The strongest currents and minimum plume extension are obtained under southwestern winds and neap tide while when northeastern winds prevail, the plume extension is at its maximum and the circulation is the weakest. We show that while neap tides allow the establishment of streamlined circulation, the spring tides induce more turbulent circulation which can favor the retention of transported elements. This latter property could not be resolved with the 5 km resolution model.