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

Abstract

Estuarine fronts are well known to influence transport of waterborne constituents such as phytoplankton and sediment, yet due to their ephemeral nature, capturing the physical driving mechanisms and their influence on stratification and mixing is difficult. We investigate a repetitive estuarine frontal feature in the Snohomish River Estuary that results from complex bathymetric shoal/ channel interactions. In particular, we highlight a trapping mechanism by which mid-density water trapped over intertidal mudflats converges with dense water in the main channel forming a sharp front. The frontal density interface is maintained via convergent transverse circulation driven by the competition of lateral baroclinic and centrifugal forcing. The frontal presence and propagation give rise to spatial and temporal variations in stratification and vertical mixing. Importantly, this front leads to enhanced stratification and suppressed vertical mixing at the end of the large flood tide, in contrast to what is found in many estuarine systems. The observed mechanism fits within the broader context of frontogenesis mechanisms in which varying bathymetry drives lateral convergence and baroclinic forcing. We expect similar trapping-generated fronts may occur in a wide variety of estuaries with shoal/channel morphology and/or braided channels and will similarly influence stratification, mixing, and transport.

Dynamics and kinematics of an estuarine network

“…While they have been previously associated with river confluences (Best, 1987;Rhoads & Kenworthy, 1995), such fronts may be quite common in estuaries. The front described in this chapter as well as similar fronts described by Redbourn (1996) and Giddings et al (2012) demonstrate that such fronts form in estuaries at junctions between channels as well as between a channel and side embayment. The buoyancy gradients across these fronts are induced by different sources of salinity, or by a tidal velocity phase-shift.…”

Turbulent and numerical mixing in a salt wedge estuary: Dependence on grid resolution, bottom roughness, and turbulence closure

“…Complex bathymetry, energetic currents, and strong density gradients make estuaries particularly challenging to represent with circulation models. Bathymetric variability directly affects water column dynamics by inducing frontogenesis, lateral circulation, and spatial gradients in mixing and baroclinic forcing [ Ralston et al ., ; Giddings et al ., ; Geyer and Ralston , ]. To simulate circulation and transport processes, hydrodynamic models must resolve the spatial variability explicitly in the model grid, or else parameterize unresolved processes such as small‐scale mixing.…”

Numerical Investigation of Baroclinic Channel‐Shoal Interaction in Partially Stratified Estuaries

“…The front trapping mechanism identified here caused by the shoaling and the subsequent vertical mixing of the salinity front can serve as an important mechanism for interfacial exchanges in estuarine systems where the main channel is flanked by shallow shoals. This type of trapping with a baroclinic nature differs from tidal trapping caused by barotropic processes, such as the differential phasing between the main channel and its side embayments or braching channels (Fischer et al, ; Giddings et al, ; MacVean & Stacey, ). For estuaries with wide shoals that are subject to strong buoyancy forcing, this process of generation and the subsequent shoalward movement of the trapped patch of salt as illustrated in Figure repeats itself in each tidal cycle, resulting in a series of local salinity maxima on the shoal.…”

“…Particular attention will be paid to answer the following outstanding questions: How does baroclinic forcing drive lateral circulations across the channel‐shoal interface? While the hydrodynamics of channel‐shoal estuaries has received some attention (Collignon & Stacey, , ; Giddings et al, ; Hoang et al, ; Lacy et al, ), a systematic investigation of the density‐driven interfacial dynamics and on‐shoal lateral exchange is lacking. For example, San Francisco Bay's distinct subtidal and intertidal mudflats have been thought to be important zones for phytoplankton production.…”