COI Funded Project: A Simple, General Model of Estuarine Plume Mixing
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.