The relevance of frontal mixing to the evolution of an energetic tidal river plume

A potential energy budget based on observations of the Columbia River tidal plume is used to assess the importance of frontal mixing processes on the evolution and ultimate fate of the tidal plume water mass. The structure and turbulent quantities of the stratified/sheared plume core and frontal regions were captured by a microstructure profile that continuously sampled during repeated shipboard transects through the front as the tidal plume propagated offshore. The front of the tidal plume is observed to rapidly evolve as it propagates offshore, and depending on the tidal forcing and ambient shelf conditions, a large frontal head wave can form with isopycnal displacements on the order of 40m. The large amplitude fronts exhibit highly elevated levels of turbulent kinetic energy (TKE) dissipation, ε, on the order of 10^-3 W/kg, which exponentially decays with increasing distance from the front until a steady value is reached in the stratified/sheared core of the tidal plume. Using the observations, parameterizations for the turbulent buoyancy flux in the frontal and plume core regions are developed which are then utilized to estimate the work done by the turbulent mixing in the respective plume regions against the buoyancy input from the river discharge. It is found that at maximum ebb and during periods of large amplitude fronts the instantaneous work done by the plume core and frontal mixing can exceed the instantaneous potential energy flux into the tidal plume from the river discharge. However, once integrated over the ebb tidal period, the change in the potential energy of the tidal plume due to mixing in the frontal and plume core regions account for 8% and 20% of the potential energy input from the river discharge, respectively. Implications relating to the structure of the resulting buoyant water mass at the end of ebb and the structure of the far-field/coastal current are discussed.