The Relative Importance of Wind Straining and Gravitational Forcing in Driving Exchange Flows in Tidally Energetic Estuaries

Abstract

In straight tidal estuaries, residual overturning circulation results mainly from a competition between gravitational forcing, wind forcing, and friction. To systematically investigate this for tidally energetic estuaries, the dynamics of estuarine cross sections is analyzed in terms of the relation between gravitational forcing, wind stress, and the strength of estuarine circulation. A system-dependent basic Wedderburn number is defined as the ratio between wind forcing and opposing gravitational forcing at which the estuarine circulation changes sign. An analytical steady-state solution for gravitationally and wind-driven exchange flow is constructed, where tidal mixing is parameterized by parabolic eddy viscosity. For this simple but fundamental situation, is calculated, meaning that the up-estuary wind forcing needs to be 15% of the gravitational forcing to invert estuarine circulation. In three steps, relevant physical processes are added to this basic state: (i) tidal dynamics are resolved by a prescribed semidiurnal tide, leading to caused by tidal straining; (ii) lateral circulation is added by introducing cross-channel bathymetry, smoothly increasing from 0.47 (flat bed) to 1.3 (parabolic bed) due to an increasing effect of lateral circulation on estuarine circulation; and (iii) full dynamics of a real tidally energetic inlet with highly variable forcing, where results from a two-dimensional linear regression.

Oceanographic Variability induced by Tides, the Intraseasonal Cycle and Warm Subsurface Water intrusions in Maxwell Bay, King George Island (West-Antarctica)

“…2a as long as the barotropic tide current is vertically sheared. However, even though tidal straining has been described (and modelled) for several estuaries 9,10,64,66–69 , it has yet to be studied in fjords.…”

Inversions of Estuarine Circulation Are Frequent in a Weakly Tidal Estuary With Variable Wind Forcing and Seaward Salinity Fluctuations

“…Contrary to their study, the Warnow estuary is a micro‐tidal estuary with typical values of U ∗ found to be smaller than 1 cm s −1 (Figure 9c) and thus an order of magnitude smaller than friction velocities found in tidally energetic estuaries, resulting in much larger values of the Si, Ts, and . However, results show that the non‐dimensional exchange flow , presented in a Ts‐Si parameter space, can still be distinguished between positive and negative circulation, shown in red and blue in Figure 10a, as suggested by Lange and Burchard (2019). The critical condition for cancellation of estuarine circulation by opposing wind stress and gravitational forcing separates both states, indicated by a bold line.…”

“…Negative estuaries, as those existing, for example, in arid regions characterized by strong evaporation, have a reversed longitudinal salinity gradient as, for example, the Persian Gulf (Johns et al., 2003) or the Spencer Gulf in south Australia (Nunes & Lennon, 1986). Temporary inverted circulation in classical estuaries can also be observed in the presence of strong landward wind (Lange & Burchard, 2019; Scully et al., 2005). Observations by Scully et al.…”

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

“…All along‐channel ( x ) gradients vanish except for the longitudinal barotropic gradient and the longitudinal salinity (buoyancy) gradient. Such lateral‐vertical GETM applications have been employed in several previous studies where longitudinal uniformity is assumed (Burchard et al, ; Hofmeister et al, ; Lange & Burchard, ; Umlauf et al, ; Schulz et al, ). The external physical drivers include a constant longitudinal buoyancy gradient and a longitudinal barotropic pressure gradient , which is constant in space but periodically varies in time.…”