Frontogenesis at Estuarine Junctions

Abstract

Observations of Newark Bay, a sub-estuary network characterized by multiple junctions, reveal that fronts are generated by tidal flow through transitions in channel geometry. All fronts substantially contribute to along-channel estuarine heterogeneity, and most are associated with both changes in channel geometry and tidal velocity phase-shifts. A lift-off front forms at the mouth of the sub-estuary during ebb tide in response to the abrupt seaward channel expansion. While forming, the front is enhanced by a tidal velocity phase-shift; flood tide persists in the main estuary until 90 minutes after the start of ebb tide in the sub-estuary. A second lift-off front forms during ebb tide at a channel-side-channel junction and is enhanced by a lateral baroclinic circulation induced by baroclinic and barotropic tidal velocity phaseshifts between the main channel and side channel. The lateral circulation also bifurcates the along-channel ebb flow at the surface, generating a surface front above the lift-off front. At the head of Newark Bay, a second surface front forms during ebb tide at the confluence of two tributary estuaries. This confluence front is rotated across the mouth of the primary fresh water source by high velocities from the adjacent tributary estuary and is maintained through much of ebb tide by lateral straining and mixing. Although the overall stratification of Newark Bay would categorize it as a partially-mixed estuary, the fronts divide the density structure of the sub-estuary into a series of nearly homogeneous segments---a characteristic that is more often associated with fjords.

Accumulation, transformation and transport of microplastics in estuarine fronts

“…2a,b). Although termed 'shear' fronts, other mechanisms might also contribute to the frontogenesis, such as confluence 41 , convergence 16 and heterogeneous cross-channel distribution of tidal mixing 42 . Axial convergence fronts are a type of shear front because they are triggered by the transverse shear of tidal currents 39 .…”

Mechanisms of Salt Overspill at Estuarine Network Junctions Explained With an Idealized Model

“…Intratidal processes are not explicitly captured by the subtidal model. However, the tidal phase difference at the junction between different channels can be important in causing a subtidal transport of salt (Corlett & Geyer, 2020). The width-averaging makes that the currents and salinity in the model do not have a lateral structure, but this can be important for overspill in wide channels, as is shown for example, in Wu et al (2006) and Xue et al (2009).…”

Exchange Flows in Tributary Creeks Enhance Dispersion by Tidal Trapping

“…The enhancement of the tidal trapping dispersion by the exchange flow in the creeks represents a mechanism which may play an important role in other multi-channel estuary systems, such as salt marsh tidal creek networks like the Coos Estuary (Conroy et al 2019) and the Plum Island Sound Estuary (Vallino and Hopkinson 1998), as well as in urbanized estuaries with various human-made ports and side channels like Newark Bay (Corlett and Geyer 2020). However, the impact of the exchange flow in the side channels on dispersion in the main channel will depend strongly on the geometry of the side channel-if it is too small, then the volumetric exchange with the main channel will be minimal and thus the total contribution to the dispersive salt flux will be negligible, but if it is too large, then the tidal velocities can become similar to the main channel and thus the axial salinity gradient-which drives the exchange flow in the side channel-will not be significantly enhanced.…”