THE COLUMBIA RIVER PLUME STUDY - SUBTIDAL VARIABILITY IN THE VELOCITYAND SALINITY FIELDS

A comprehensive study of the strongly wind driven midlatitude buoyant plume from the Columbia River, located on the U.S. west coast, demonst rates that the plume has two basic structures during the fall/winter s eason, namely, a thin (similar to 5-15 m), strongly stratified plume t ending west to northwestward during periods of southward or light nort hward wind stress and a thicker (similar to 10-40 m), weakly stratifie d plume tending northward and hugging the coast during periods of stro nger northward stress. The plume and its velocity field respond nearly instantaneously to changes in wind speed or direction, and the wind f luctuations have timescales of 2-10 days. Frictional wind-driven curre nts cause the primarily unidirectional flow down the plume axis to vee r to the right or left of the axis for northward or southward winds, r espectively. Farther downstream, currents turn to parallel rather than cross salinity contours, consistent with a geostrophic balance. In pa rticular, during periods when the plume is separated from the coast, c urrents tend to flow around the mound of fresher water. At distances e xceeding about 20 km from the river mouth, the along-shelf depth-avera ged flow over the inner to midshelf is linear, and depth-averaged acce leration is governed to lowest order by the difference between surface and bottom stress alone. In this region, along-shelf geostrophic buoy ancy-driven currents at similar to 5 m (calculated from surface densit y) and along-shelf geostrophic wind-driven currents (computed from a d epth-averaged linear model) are comparable in magnitude (similar to 10 -25 cm s(-1)).