The initial evolution of a buoyant plume

The dynamics of river plumes in a stratified non-tidal sea is considered on the basis of a linear analytical theory and simulations with a numerical primitive equations model based on the GFDL-general circulation model with free surface. The response to an onset of river runoff consists basically of two parts: The formation of a freshwater bulge right in front of the river mouth and the set up of a coastal current by Kelvin waves. The properties of the Kelvin waves depend on the stratification of the sea. For an unstratified sea the baroclinic Kelvin wave modes are missing and the spreading of the river water is mainly confined to the near field and a slow alongshore propagation of the freshwater, while for a stratified sea the baroclinic Kelvin waves propagate alongshore and establish the far field response. After about 15–20 days of simulation time a secondary bulge develops downstream. A clue for the understanding of the plume structure is the weakly nonlinear effect due to the changes of the stratification by the spreading of the buoyant freshwater. If wind forcing is taken into account, the plume pattern is strongly affected by the on- or off-shore Ekman transport.