Axial Wind Effects on Stratification and Longitudinal Salt Transport in an Idealized, Partially Mixed Estuary*

Abstract

A 3D hydrodynamic model [Regional Ocean Model System (ROMS)] is used to investigate how axial wind influences stratification and to explore the associated longitudinal salt transport in partially mixed estuaries. The model is configured to represent a straight estuarine channel connecting to a shelf sea. The results confirm that wind straining of the along-channel salinity gradient exerts an important control on stratification. Two governing parameters are identified: the Wedderburn number (W) defined as the ratio of wind stress to axial baroclinic pressure gradient force, and the ratio of an entrainment depth to water depth (hs/H). Here W controls the effectiveness of wind straining, which promotes increases (decreases) in stratification during down-estuary (up-estuary) wind. The ratio hs/H determines the portion of the water column affected by direct wind mixing. While stratification is always reduced by up-estuary wind, stratification shows an increase-then-decrease transition when down-estuary wind stress increases. Such transition is a result of the competition between wind straining and direct wind mixing. A horizontal Richardson number modified to include wind straining/mixing is shown to reasonably represent the transition, and a regime diagram is proposed to classify the wind’s role on stratification. Mechanisms driving salt flux during axial wind events are also explored. At the onset and end of the wind events, barotropic adjustment drives strong transient salt fluxes. Net salt flux is controlled by the responses of subtidal shear dispersion to wind forcing. Moderate down-estuary winds enhance subtidal shear dispersion, whereas up-estuary winds always reduce it. Supporting observations from upper Chesapeake Bay are presented.

Investigation of saline water intrusions into the Curonian Lagoon (Lithuania) and two-layer flow in the Klaipėda Strait using finite element hydrodynamic model

“…For example, Valle-Levinson et al (2004) showed, from measurements, how the balance between wind stress and barotropic pressure gradients justifies the vertical integrated dynamics for a specific strait in the Chilean Sea. In order to investigate the wind effects on stratification of waters in the Klaipėda Strait and type of flow regime, two governing dimensionless parameters are identified: the Wedderburn number (W ), defined as the ratio of wind stress to axial baroclinic pressure gradient force, and the ratio of the buoyancy layer depth to water depth (h b /H ) (Chen and Sanford, 2009). The Wedderburn number (Monismith, 1986) describes the relative importance between the wind-driven circulation and the baroclinic pressure gradient along the strait: where τ wx is the along-strait wind stress (positive up-strait, e.g., towards the lagoon), L is the length of the strait (14 km), ρ is the density change over L, g is the gravitational acceleration, and H is the averaged depth of the strait (11 m).…”

“…Previous investigations of the lagoons have mainly focused on tidal-and winddriven exchanges across the inlets (Stommel and Farmer, 1952;Wong, 1991;Geyer and Signell, 1992;Churchill et al, 1999;Hench et al, 2002;. However, more and more attention in recent years is paid to circulation dynamics and salt balance inside the estuarine lagoons (Reyes-Hernandez and Valle-Levinson, 2010;Chen and Sanford, 2009;Jia and Li, 2012a, b;Kim and Park, 2012;Li and Li, 2012). Estuarine lagoons with complex interactions between biotic and abiotic components depend on the water exchange between the lagoon and the sea.…”

Effect of winds and waves on salt intrusion in the Pearl River Estuary

“…which is much smaller than the critical value of ~1 (0.85 in Chen and Sanford, 2009) and suggests that the wind straining effect dominates over the wind mixing. The Wedderburn numbers for the mean down-estuary (N, NE) winds are 0.14 and 0.08, respectively, again indicating that the wind straining dominates.…”

“…The down-estuary wind can enhance stratification and estuarine circulation via wind straining, and can also simultaneously increase vertical mixing via wind mixing; therefore its effect on salt transport reflects the competition between these two components. However, the up-estuary wind increases vertical mixing and decreases estuarine circulation, consequently reducing the salt intrusion (Chen and Sanford, 2009;Jia and Li, 2012).…”

Drivers of Barotropic and Baroclinic Exchange through an Estuarine Navigation Channel in the Mississippi River Delta Plain

“…First, they drive a two-layer circulation that opposes gravitational circulation, in which surface currents flow into the estuary in the same direction as the wind and near-bottom currents return water to the ocean. Additionally, the water column is mixed through direct wind mixing, where turbulence generated at the air-sea interface is transferred down through the water column [54]. During all tidal cycles sampled, the dominant salt transport term was F 0 ( Table 1), indicating that fluxes of salt were primarily driven by a combination of freshwater discharge through the HNC and wind-driven changes in storage over the course of the tidal cycle.…”