Structure and Propagation of the Outer Front of the Columbia River Plume

As part of a study of juvenile salmon survival, the structure and propagation of the outer Columbia River plume front was examined using vessel observations and Synthetic Aperture Radar (SAR) satellite data. During low-to-moderate riverflow and typical anti-cyclonic summertime winds, two vessels followed successive tidally-pulsed greater-ebb plume fronts as they propagated radially to sea. Shipboard and helicopter-derived front tracking maps showed similar front propagation patterns to those seen in SAR observations. The plume was strongly stratified, with a vertical salinity gradient typically ranging from 2-4 m^-1, yet rapidly mixing, with a consistent supercritical internal Froude number. Within and immediately behind the front, ADCP current profiler measurements indicated that velocities were convergent above and within the pycnocline, but divergent in the lower water column. SAR and drifter data also confirmed convergence in surface currents. This convergence likely explains observations of elevated concentrations of certain surface-seeking salmon prey within the front. Although there was convergence in the across-front velocity, the shear in along-front velocity was strong, providing a mechanism for dispersing organisms along the front line. At 8 h past higher-high water (HHW), in 40 m deep water, the front was characterized by a strong surface across-front salinity difference, ~10. Limited ADCP observations of vertical velocities were obtained when the ship cruised within the front line, showing downwelling at up to 0.35 m s^-1. Elevated acoustic backscatter and velocities were seen in the bottom boundary layer directly below the front, suggesting that internal bore velocities may have been strong enough to locally resuspend sediment. By 12 h past HHW, front propagation had slowed dramatically, the across-front salinity difference had declined to ~5, and the front location implied a mean frontal velocity of 0.8 m s^-1 since frontogenesis. Maximum downwelling was O(0.10 m s^-1), and there was no longer any indication of an influence near the seabed. Surface drifter measurements indicated front-normal convergence was occurring at velocities of ~0.25 m s^-1 from both sides of the front. SAR backscatter and shipboard observations were consistent with a rotary, bore-like vertical circulation, with downwelling at the frontline and upwelling behind the front. These observations suggest that a substantial fraction of total plume mixing and nutrient entrainment may occur in highly-nonlinear features like this bore-like head.