A Simple, General Model of Estuarine Plume Mixing

Ken Brink, Physical Oceanography


Project Funded 2007:

Freshwater outflows from land into the coastal ocean are extremely common and diverse features globally. Only in a few desert or frozen regions are these features not present. These discharges are important because they strongly affect biological and chemical processes over the shelf. For example, excess nutrients in the Mississippi outflow drive eutrophication (excesses growth of microscopic plants) and consequent oxygen depletion over the Louisiana shelf.  The salinity distribution within the outflow determines how quickly the plume waters move, and apparently how far alongshore they can penetrate. Further, the dilution of the plume waters by mixing with ambient shelf waters strongly affects biological and chemical processes in the plume.

One intriguing aspect of buoyancy currents over the continental shelf is how, for a very wide variety of settings (e.g., the Columbia River, Chesapeake Bay or Gulf of Maine rivers), the dilution of discharge waters is about the same: the plume waters mix to be about 1-4 salinity units fresher than ambient. It is unusual to find examples of water fresher than that away from the immediate mouth of an estuary. This constancy remains a puzzle to coastal ocean scientists.

This project will use a simple model of mixing in a buoyancy current so as to characterize the amount of dilution. Preliminary results show that there is a surprisingly simple and general formulation for this problem. The model has one unknown parameter, and I will attempt to estimate it through idealized primitive equation numerical model runs that do include an accepted, realistic parameterization of vertical mixing. I hope and expect to obtain general answers for the enduring puzzle of the relative constancy of mixing in buoyancy currents. The model results could provide simple predictive tools for estimating dilution of river outflows, hence concentrations for nutrients and other dissolved materials.

This project is directly relevant to the Coastal Ocean Institute theme ”the dynamic land/sea margin” in that it deals with the transport and exchange between salt- and freshwater systems, and that it is critical to the dynamics of continental shelves and estuaries.