Mechanisms of Estuarine Salt-Plug Formation by an Along-Shelf Buoyant Current: A Numerical Model Approach

While estuarine salt plugs can develop worldwide in basins adjacent to buoyant coastal currents, their formation has been scarcely documented. This study aims to investigate a mechanism for salt-plug formation that disregards evaporation processes but involves a buoyant coastal current modified by wind stresses. A numerical model, Delft3D, is used to study the salt-plug formation in an idealized bay connected to the ocean by a single inlet. Inspired by recent observations, the numerical experiments simulate eight different scenarios of tidal and wind forcing under the influence of an along-shelf buoyant current. Results show salt-plug formation that induces inverse exchange at the inlet, with inflow at the surface and outflow underneath. This exchange circulation is modified by wind action. The persistence of the salt plug depends on tidal flushing and on wind forcing. Two numerical experiments with nonstationary buoyant currents and nonstationary winds indicate that: (a) a salt plug forms when a buoyant current is active on the shelf and traps salty water that enters the bay during times of buoyant current relaxation; (b) the presence of the buoyant current induces an inverse circulation at the inlet, modified by the wind action; and (c) onshore and then downwelling winds enhance the inverse circulation at the inlet, while offshore and then upwelling winds stall it. Bay flushing times increase due to the presence of the salt plug. This study represents an initial attempt to identify the role of wind and buoyant coastal currents on salt-plug formation.