Search for: All records

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.

Along the Atlantic coast of the United States, interannual sea‐level variations of up to 20 mm are superimposed regionally upon the global average sea‐level rise (~3 mm/year) from human‐caused global warming. These variations affect the degree of coastal flooding and related damage during the highest annual tides. Interannual sea‐level variations have been attributed to several atmospheric and oceanographic processes. In the present analysis, detrended tide gauge data isolate >5‐year interannual variations. These variations can be reliably reconstructed (>77% of the variance explained) with Fourier coefficients that have frequencies related to lunar orbit (nodalandapsidalprecessions) combined withsolar activity. Although a causal relationship between astronomical forcing and extreme sea levels remains elusive, the reconstructions may provide an effective method for projections of extreme sea levels. Two reconstructions project that anomalously high sea levels may occur in the late 2020s, mid‐2050s, early 2060s, early 2070s, and late 2090s.

Dye‐release and numerical modeling experiments are conducted to investigate horizontal dispersion in a partially mixed estuary. Longitudinal dispersion of a dye patch shows marked flood‐ebb asymmetry in the first two tidal cycles after a dye release, with most of the dispersion occurring during ebb tides. There are large differences in the dispersion rate between spring and neap tides. Due to increased vertical mixing during spring tides, a dye patch quickly extends from the bottom to the surface and is exposed to the vertical shear in the entire water column, enhancing longitudinal dispersion. In contrast, most of the dye patch is limited to the bottom few meters during the neap tides. Although decreased vertical mixing during neap tides facilitates longitudinal dispersion, the vertical shear across the thin dye patch in the bottom layer is much weaker than the full water column shear in spring tides. Decreased shear leads to reduced longitudinal dispersion. After four tidal cycles, the second moment of the dye patch increases with time in the along‐channel direction at a power of between 2 and 3. The longitudinal dispersion rate varies as the four thirds power of the dye patch size, indicating scale‐dependent diffusion.