The bulge circulation in the Columbia River plume

While most numerical and laboratory models of coastal river inflows result in the generation of a large anticyclonic bulge near the mouth of the river, evidence of such a feature in field observations is scant. Here, results from a series of approximately shore-parallel transects 20 km west of the mouth of the Columbia River are presented that provide a detailed description of the bulge circulation in the plume during a period of moderate wind stress. The transect data include velocity and density measurements in the upper 30 m of the water column, acquired with a vessel-mounted ADCP and a CTD towed on a vertically undulating TRIAXUS towfish, respectively. Sampling spanned more than 19 h of the mixed semidiurnal tidal cycle, and the anticyclonic circulation persisted throughout this period. The core of the bulge circulation, defined as the central region where the azimuthal velocity varies linearly, is approximately 5 m deep and 22 km in diameter, and the entire bulge circulation is 30–40 km in diameter. The observations from the Columbia plume agree well with the structure and scales proposed in laboratory and numerical model studies. The existence of the core region of constant vorticity agrees with laboratory studies that also show the bulge to be in solid-body rotation. The average rotation rate in this region is approximately -0.8f-0.8f, close to the zero potential vorticity limit. Estimates of each of the terms in the radial momentum equation confirm that the bulge is in gradient-wind balance. Predictions based on previous analytical and scaling theories and observations from the Columbia plume suggest that the accumulation of freshwater in the bulge should reduce the northward coastal current flux to approximately 35% of the river discharge. This agrees with preliminary estimates of northward flux in the coastal current during the same period.