Collaborative Research: Using Salinity Variance to Link Estuarine Mixing and Exchange Flow

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The variance of salinity is one of the most important quantities defining an estuary. Horizontal salinity variance provides the driving force for the gravitational circulation, and vertical variance quantifies stratification, which strongly affects and is affected by the shear, turbulence and mixing. The salinity variance balance has great promise as an approach to understanding the interaction of mixing and exchange flow in estuaries. This equation has been effectively exploited in the turbulence community to quantify mixing, but it has received surprisingly little attention in context with estuarine dynamics, in spite of its fundamental importance. This project will analyze the salinity variance balance in a variety of estuaries that span a broad swath of parameter space, using realistic and idealized numerical models. The insights gained from the analysis will provide a more systematic understanding of estuarine dynamics, using a common framework to compare rates and mechanisms across the estuarine parameter space. The analysis approach will also have application to exchange flows and mixing in the coastal and global ocean. This project will also support a graduate student at each of the two institutions and contribute to their training. The PIs also plan to teach a 5-week summer class for graduate students entitled "Estuarine Dynamics: Observations and Realistic Models." The class will involve 12-15 graduate students interested in estuarine processes. The students will gain hands-on field experience, and they will also apply and analyze realistic models, thereby developing a sense of the challenge and synergy that comes from bringing together observations and models.This project will develop the mathematical framework of the salinity variance to quantify the integral relationships between exchange flow and mixing, and then apply those relationships to diverse estuarine settings. In addition to these integral relationships, the analysis will examine the local exchange between horizontal and vertical variance via straining, and determine the consequences for temporal and spatial variations of stratification and mixing. Here "mixing" is defined as the irreversible loss of salinity variance by turbulent and molecular processes. The analysis will address these fundamental questions: Is exchange flow controlled by mixing, or vice versa? How do mixing and exchange flow respond to variations in tidal amplitude in different estuarine regimes? How does estuarine geometry affect mixing and exchange flow? How does straining by estuarine and tidal shear vary in different regimes, and how does straining modulate the exchange between horizontal and vertical variance and thereby enhance or suppress mixing? The answers to these questions will lead to a predictive framework for the variability of estuarine mixing and exchange flow in relation to the estuarine geometry and the dominant forcing parameters; river discharge and tides.