Salt flux into coastal river plumes: Dye studies in the Delaware and Hudson river outflows

The results of ten dye tracer experiments conducted in 2003-2006 to study the dispersion of the outflow of the Delaware and Hudson Rivers are presented. A fluorescent dye tracer was used to track the river plume and to measure directly the salt flux into the plume. A variety of flow regimes were encountered. During strong upwelling events, a salt flux of ~3 ÷ 10^-4 kg m s^-1 at the leading edge of the plume implies a vertical diffusivity of K_z ~ 3 × 10^-4 m^-2 s^-1. Comparable salt flux was measured at the leading edge of a buoyancy-driven coastal current with K_z ~ 6.3 × 10^-4 m^-2 s^-1. For weaker wind events K_z was ≤10^-4 m^-2 s^-1. Using a gradient Richardson number (Ri), these observations were replicated by a 1-D model of vertical salt flux to within a factor of 2. Upwelling events are the most efficient mechanism for dispersing the river plume water over the coastal shelf because the plume's offshore displacement is combined with a horizontal diffusivity measured to be ~150 m^-2 s^-1 over the two-day period of each experiment.