Near-Field Frontal Observations From a High-Volume River Plume

We present observations of very strong convergence, vertical velocities, mixing and bottom boundary layer impacts associated with the leading edge front of the tidally-pulsed Columbia River plume. During four successive greater-ebb spring tides in May 2001, with typical summertime upwelling-favorable winds and riverflow (4900 m³ s^-1), shipboard and helicopter-derived front tracking maps show front propagation patterns similar to those generally seen in Synthetic Aperture Radar (SAR) observations. Shipboard observations during the latter part of ebb tide (5 to 8 h past high water) are consistent with an inertial force balance and a rotary, bore-like vertical circulation. Downwelling velocities are 5-35 cm s^-1, extending down to the bed below the frontline, reaching our deepest measurements of 70 m. Based on comparisons between oceanic and plume T-S diagrams, the mixing layer extends down to an average depth of 18 m in the region from 50-400 m behind the frontline. We estimate mean mixing parameters over this distance and depth using instability length-scales in ∼200 CTD density profiles (a "Thorpe scale" analysis). The mean dissipation, eddy diffusivity, and Thorpe overturn scale are 2 x 10^-4 W kg^-1, 6 x 10^-2 m² s^-1 and 0.79 m respectively, and decrease with distance behind the front. These observations corroborate the conclusion, from prior laboratory studies and observations of low-volume river plumes, that the leading-edge front plays a dominant role in the flux of buoyancy and other constituents between ocean and plume.