Longitudinal Versus Lateral Estuarine Dynamics and Their Role in Tidal Stratification Patterns in Lower South San Francisco Bay

Abstract

The dynamics of shoal‐channel estuaries require consideration of lateral gradients and transport, which can create significant intratidal variability in stratification and circulation. When the shoal‐channel system is strongly coupled by tidal exchange with mudflats, marshes, or other habitats, the gradients driving intratidal stratification variations are expected to intensify. To examine this dynamic, hydrodynamic data were collected from 27 January 2017 to 10 February 2017 in Lower South San Francisco Bay, a small subembayment fringed by extensive shallow vegetated habitats. During this deployment, salinity variations were captured through instrumentation of six stations (arrayed longitudinally and laterally) allowing for mechanisms of stratification creation and destruction to be calculated directly and compared with observed time variability of stratification at the central station. We present observation‐based calculations of longitudinal straining, longitudinal advection, lateral straining, and lateral advection. The time dependence of stratification was observed directly and calculated by summing measured longitudinal and lateral mechanisms. We found that the stratification dynamics switch between being longitudinally dominated during the middle of ebb and flood tides to being laterally dominated during the tidal transitions. This variability is driven by the interplay between tidally variable lateral density gradients and turbulent mixing. Relatively constant along‐estuary density gradients are differentially advected during flood and ebb tides, resulting in maximal lateral density gradients around tidal transitions. Simultaneous decrease in turbulent mixing at slack tides allows lateral density‐driven exchange to stratify the estuary channel at the slack after flood. At the end of ebb, barotropic forcing drives negatively buoyant shoal waters toward the channel.

Areal parameter estimates from multiple datasets

“…We note that a number of the autocorrelation estimates using earthquake signals remain discordant, even though they have influenced the construction of the Moho surface. The physical model used to extract these results enforces a sharp discontinuity [7], and the differences from the interpreted surface, where present, provide an indication of the thickness of the crust-mantle transition. We also display in figure 5 b the topography across the region, which is relatively subdued.…”

“…Since 2011 the number of Moho estimates in Australia has grown significantly, with a strong continuing programme of full-crustal reflection profiling and the advent of new methods such as the exploitation of stacked station auto-correlograms [6,7]. In many places this means that there are multiple values for the depth to Moho based on different physical assumptions.…”

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

“…Drowned‐river estuaries are usually composed of a deep channel laterally bounded by shallow shoals. Typical examples exhibiting such channel‐shoal morphology include the Chesapeake Bay (e.g., Valle‐Levinson & Lwiza, ), the Wadden Sea (e.g., Purkiani et al, ), and the San Francisco Bay (e.g., Hoang et al, ). The channel‐shoal interface in partially stratified estuaries is a dynamically important region where complicated lateral circulations can develop (Collignon & Stacey, ).…”

“…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.…”

“…Since the present study focuses on the baroclinic forcing of lateral circulation and transport, lateral barotropic forcing arising from the change of water surface elevation is neglected by making a rigid‐lid assumption. This limitation may be important especially on the shallow shoal, where strong lateral barotropic transport could be observed when the shoal is ventilated at low water (Hoang et al, ). Furthermore, the change of shoal depth also indirectly affects the lateral baroclinic forcing and thus transport.…”