The Relation between Exchange Flow and Diahaline Mixing in Estuaries

Abstract

Estuaries with excess freshwater discharge are characterized by inflow of saline water from the ocean and outflow of brackish water toward the ocean. The intensity of this so-called estuarine circulation is known to be connected to salinity mixing inside the estuary. Here, the water mass transformation process of mixing is conveniently defined as the destruction of salinity variance within the estuarine volume. In the present study, we derive a new relation that shows how the estuarine exchange flow and the mixing inside the estuary determine each other. The result is a quantitative relation between the inflow volume at a fixed transect across the estuary and the distribution of the diahaline mixing inside the estuary. The concept of dividing salinities between inflowing and outflowing transports is used here. More specifically, the estuarine exchange flow equals the derivative of the mixing (with respect to salinity) on the estuarine part of the dividing salinity isohaline. We show for two realistic example estuaries how the exchange flow and the mixing are spatially distributed on the dividing salinity isohaline. Those estuaries include a channelized tidal estuary and a nontidal industrialized estuary. While the former shows a basic two-layer division between inflow and outflow, the latter has a more complex three-layer structure, due to offshore upwelling and downwelling dynamics. The new theory provides the recipe for analyzing the relation between mixing and exchange flow in any estuary and helps to identify the regions where the mixing occurs that is consistent with the exchange flow.