Abstract
AbstractAsymmetric tidal turbulence (ATT) strongly influences estuarine health and functioning. However, its impact on the three-dimensional estuarine dynamics and the feedback of water motion and salinity distribution on ATT remain poorly understood, especially for short estuaries (estuarine length ≪ tidal wavelength). This study systematically investigates the abovementioned interactions in a short estuary for the first time, considering periodically weakly stratified conditions. This is done by developing a three-dimensional semi-analytical model (combining perturbation method with finite element method) that allows a dissection of the contributions of different processes to ATT, estuarine circulation, and salt transport. The generation of ATT is dominated by (i) strain-induced periodic stratification and (ii) asymmetric bottom-shear-generated turbulence, and their contributions to ATT are different both in amplitude and phase. The magnitude of the residual circulation related to ATT and the eddy viscosity–shear covariance (ESCO) is about half of that of the gravitational circulation (GC) and shows a “reversed” pattern as compared to GC. ATT generated by strain-induced periodic stratification contributes to an ESCO circulation with a spatial structure similar to GC. This circulation reduces the longitudinal salinity gradients and thus weakens GC. Contrastingly, the ESCO circulation due to asymmetric bottom-shear-generated turbulence shows patterns opposite to GC and acts to enhance GC. Concerning the salinity dynamics at steady state, GC and tidal pumping are equally important to salt import, whereas ESCO circulation yields a significant seaward salt transport. These findings highlight the importance of identifying the sources of ATT to understand its impact on estuarine circulation and salt distribution.
Highlights
Vertical turbulent fluxes of momentum and salt are of fundamental importance to estuarine water motion and mass transport, strongly influencing the estuarine morphology, biology, and ecology
The different asymmetric tidal turbulence and eddy viscosity–shear covariance (ESCO) circulation patterns caused by strain-induced periodic stratification (SIPS) and bottom-shear-generated turbulence (BGT) shown in this study indicate that the variable ESCO circulation patterns found in previous studies may be related to different processes being dominant in different systems
Focusing on asymmetric tidal turbulence (ATT) at the semidiurnal (M2) tidal frequency, a semi-analytical model was developed to study the dynamic interactions between asymmetric tidal turbulence, water motion and salinity in periodically, weakly stratified estuaries
Summary
Vertical turbulent fluxes of momentum and salt are of fundamental importance to estuarine water motion and mass transport, strongly influencing the estuarine morphology, biology, and ecology. Of particular interest in the present paper are the variations of vertical eddy viscosity and diffusivity at the dominant tidal frequency and their interactions with the water motion and salt transport. The dominant tidal component of these coefficients parameterizes variations of smallscale turbulence during the tidal cycle and is responsible for flood–ebb asymmetry in small-scale turbulence. Such asymmetries were first observed by Simpson et al (1990) and will be called asymmetric tidal turbulence (ATT) hereafter
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