How estuaries work: A Charlotte Harbor example
We consider the time-averaged, estuarine circulation through a control volume analysis of its mechanical energy balance; hence the title of our paper: "How estuaries work." The venue is the Charlotte Harbor on Florida's west coast, and the analysis medium is the primitive equation ECOM3D-si model. We are motivated by an estuary classification dilemma. Being that estuarine circulation is buoyancy driven by the horizontal salinity gradient, and that the gradient increases with increasing vertical mixing as the salinity isolines rotate from horizontal to vertical, it follows that an increase in mixing should lead to an increase in circulation. Yet, at some point, intermediate between partial and well mixed, the estuarine circulation decreases. We explore this conundrum by considering the sources and sinks of mechanical energy, derived from river, tide, and wind forcing, through analyses of pressure work, work by stresses operating on the surface and the bottom, work against buoyancy through mixing and advection, and turbulence energy production by velocity component shears. How the energy partitions between work against buoyancy (that sets the driving force) and turbulence production (that serves as a brake) provides an answer. Depending on tides and winds, if the incremental rate of turbulence production exceeds that of buoyancy work, the estuarine circulation slows. A useful measure is the ratio of buoyancy work to turbulence production. As this ratio increases so does the circulation, and conversely. For the Charlotte Harbor, tides alone cause peak circulation. Weak, oscillatory winds may add to this, but with increasing wind stress the tide and wind-averaged estuarine circulation decreases. Further exploration of these effects across Rayleigh number space with varying estuarine geometries and forcing functions may lead to improved understandings of estuary workings in general.
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Document Type: Research Article
Publication date: 2003-09-01
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