Large scale buoyancy driven circulation on the continental shelf

Freshwater discharge from rivers and estuaries produces plumes and coastal currents locally. What is the fate of these coastal currents far downstream and over long time scales? The mechanism and processes related to the slow alongshelf evolution of a surface-trapped plume in the presence of an ambient flowfield are studied with the help of a three-dimensional primitive equation numerical model. As the buoyant water enters the coastal ocean, it turns anticyclonically and moves downstream in the direction of Kelvin wave propagation. Close to the source, the plume is surface-trapped. The offshore buoyancy flux in the bottom boundary layer pushes the front seaward detaching it from the coast and moving it offshore. Eventually, the cross-shelf buoyancy flux vanishes shoreward of the front and the front reaches an equilibrium depth at a particular alongshelf location. At this point, the quasi-stationary buoyancy balance is between horizontal and vertical diffusion and cross-shelf buoyancy flux. The density front has switched from a surface-trapped plume to an intermediate or surface-to-bottom type at this location. Horizontal diffusion contributes to effective horizontal mixing and helps spread freshwater offshore. The offshore penetration of buoyant water depends on three dimensionless parameters: scaled inlet volume transport, scaled breadth and scaled “diffusivity”. We develop an empirical relation for predicting maximum penetration, given these three parameters.