Numerical Investigation of Baroclinic Channel‐Shoal Interaction in Partially Stratified Estuaries. Issue 4 (13th April 2020)

• Record Type: Journal Article
• Authors: Zhou, Jian; Stacey, Mark T.; Holleman, Rusty C.; Nuss, Emma; Senn, David B.
• Is Part Of: Journal of geophysical research. Volume 125:Issue 4(2020)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2020-04-13
• Subjects: Oceanography -- Periodicals; 551.4605
• DOI: 10.1029/2020JC016135
• Languages: English
• ISSNs: 2169-9275

Abstract

Abstract: The internal, density‐driven channel‐shoal interaction in partially stratified estuaries is numerically investigated. Idealized General Estuarine Transport Model (GETM) simulations with the rigid‐lid assumption are performed in a two‐dimensional cross‐sectional mode forced by a constant longitudinal salinity gradient and an M2 tidal current. Simulation results show that within each tide cycle, the abrupt flattening of bathymetry at the channel‐shoal interface leads to the trapping of a patch of high‐salinity water mass on the shoal. The resulting local salinity maximum near the interface interacts with the differential advection by the gentle slope of shoal, driving complex density‐driven on‐shoal lateral circulations. Next, it is found that the presence of the shoal in turn enhances the longitudinal residual circulation in the channel. As a storage of estuarine water that is partially isolated from the main stream, the shoal traps saltier water that originates from the channel during slack after ebb (thus a less stratified flood), while injects fresher water into the surface of the channel during slack after flood (thus a more stratified ebb). These lateral baroclinic exchanges across the channel‐shoal interface give rise to a new mechanism for generating tidal asymmetry of eddy viscosity and shear in the channel (i.e., tidal straining circulation), which has the same orientation with the classical estuarine circulation. The interfacial salt trapping and the accompanied shoal‐induced modification of channel residual circulation are demonstrated to possess a highly localized nature, with their characteristics largely independent of shoal bathymetry far away from the channel‐shoal interface. Plain Language Summary: Drowned‐river estuaries are frequently characterized by a deep channel laterally bounded by shallow shoals with a gentle slope. Many such estuaries are experiencing impairment from nutrient over‐enrichment such as low dissolved oxygen level, massive phytoplankton blooms and water quality degradation. Previous field and modeling studies have indicated that the shallow shoals with higher light availability allow for faster phytoplankton growth and that its exchange with the channel can influence the occurrence, timing and extent of large‐scale phytoplankton blooms. We perform idealized numerical simulations of straight estuaries with distinct channel‐shoal morphology to explore the hydrodynamic processes driven by differences in fluid density inside the water body. We focus on how the salt is exchanged between channel and shoal and how the water on the shoal moves perpendicular to the main axis of the estuary. We also highlight that the presence of the shoal tends to enhance the net along‐estuary transport of water in the central channel when the oscillating tide is filtered out. The insights gained are expected to expand our knowledge of the physics of channel‐shoal estuaries at various timescales, which can aid us in making management decisions regarding estuarine ecosystems. Key Points: Abrupt flattening of bathymetry traps salt on the shoal and creates a local salinity maximum Baroclinic channel‐shoal exchange enhances longitudinal residual circulation in the channel Interfacial salt trapping and feedback of shoal to channel are highly localized processes