The Columbia River Plume Study: Subtidal variability in the velocity and salinity fields

Abstract

Abstract. A comprehensive study of the strongly wind driven midlatitude buoyant plume from the Columbia River, located on the U.S. west coast, demonstrates that the plume has two basic structures during the fall/winter season, namely, a thin (---5-15 m), strongly stratified plume tending west to northwestward during periods of southward or light northward wind stress and a thicker (---10-40 m), weakly stratified plume tending northward and hugging the coast during periods of stronger northward stress. The plume and its velocity field respond nearly instantaneously to changes in wind speed or direction, and the wind fluctuations have timescales of 2-10 days. Frictional wind-driven currents cause the primarily unidirectional flow down the plume axis to veer to the right or left of the axis for northward or southward winds, respectively. Farther downstream, currents turn to parallel rather than cross salinity contours, consistent with a geostrophic balance. In particular, during periods when the plume is separated from the coast, currents tend to flow around the mound of fresher water. At distances exceeding about 20 km from the river mouth, the along-shelf depth-averaged flow over the inner to midshelf is linear, and depth-averaged acceleration is governed to lowest order by the difference between surface and bottom stress alone. In this region, along-shelf geostrophic buoyancy-driven currents at ---5 m (calculated from surface density) and along-shelf geostrophic wind-driven currents (computed from a depth-averaged linear model) are comparable in magnitude (---10-25 cm s-•).

Determining indicator taxa across spatial and seasonal gradients in the Columbia River coastal margin

“…This class of bacteria was previously shown to be prevalent in productive environments like phytoplankton blooms (Simon et al, 1999) and upwelling zones (Alonso-Saez et al, 2007). The Columbia River plume is highly productive with seasonal upwelling supplying nutrients to the surface waters to fuel production (Hickey et al, 1998;Hickey et al, 2010). The top plume indicator was OTU 9443, a Polaribacter taxon, which is a genus of class Flavobacteria.…”

“…The biological and physical processes of these waters are complex due to variable winds, remote wind forcing, shelf width, and submarine canyons (Hickey and Banas, 2008;Hickey et al, 2010), which in turn may differentially affect the composition of bacterioplankton communities along the Oregon and Washington coasts (Fortunato and Crump, 2011). The Columbia River is the second largest river in the United States with a mean annual discharge of 7300 m 3 s À 1 (Hickey et al, 1998). This significant release of freshwater has a strong impact on the chemical, physical, and biological characteristics of the river plume and coastal ocean (Hickey et al, 2010).…”

Buoyant gravity currents along a sloping bottom in a rotating fluid

“…For example, river water discharging into saltier, and hence denser, ocean water turns cyclonically and forms a narrow buoyant gravity current that can flow hundreds of kilometres along the coast before dispersing (e.g. Munchow & Garvine 1993;Hickey et al 1998;Rennie, Largier & Lentz 1999). The characteristics and dynamics of buoyant gravity currents along a vertical wall are relatively well understood from laboratory, theoretical and numerical model studies (Stern, Whitehead & Hu 1982;Griffiths & Hopfinger 1983;Kubokawa & Hanawa 1984a, b;Griffiths 1986;Helfrich, Kuo & Pratt 1999).…”

Laboratory modelling of the effects of temporal changes of estuarine-fresh-water discharge rates on the propagation speed of oceanographic coastal currents

“…For the Columbia River Plume, Hickey et al (1998) report that the flow into the estuary varies between 3000 m 3 s −1 6 q 0 6 17 000 m 3 s −1 over a typical year. The salinity of ocean water quoted is 32 psu, corresponding to g ≈ 0.2 m s −2 .…”

“…When the spreading exceeds the Rossby deformation scale, the dynamics of the discharge are affected by the Coriolis force arising from the rotation of the earth, and the estuarine water is confined to the coastal zone where it establishes a buoyancy-driven current flowing along the coast. One typical example of such coastal currents is the Columbia River Plume (Hickey et al 1998); for further examples, refer to Thomas & Linden (2007) (hereinafter TL). Previous studies of coastal currents (e.g.…”