Observational estimates of entrainment and vertical salt flux in the interior of a spreading river plume

Abstract

Observational estimates of entrainment and vertical salt flux into the tidally pulsed Columbia River plume are presented. Drifting buoys deployed in ebbing plume water downstream of the lift‐off region are used to make measurements of velocity, salinity, and horizontal divergence in the surface layer. Drifters released near the river mouth, shoreward of the propagating ocean‐plume front, catch up to the front, consistent with classical models of plume front circulation. During their offshore transit, drifter streamwise velocities decrease while salinities increase. Assuming drifter‐measured quantities are representative of plume‐averaged quantities, conservation equations evaluated between adjacent spreading drifters are used to determine the plume depth and entrainment velocity throughout the experiment. This approach provides knowledge of the mixing involved between the active lift‐off region and the offshore ocean‐plume front as the flow evolves. Because the surface drifters do not downwell at the ocean‐plume front, we do not estimate mixing in that region. We find that entrainment and vertical salt flux are largest near the river mouth, as expected, with typical values of O(10−3 m s−1) and O(10−2 practical salinity units (psu) m s−1), respectively, and that these quantities steadily decrease as the drifters move offshore. Profiles of water properties following the drifters provide knowledge of vertical stratification (0.5–4 psu m−1 at the plume base) allowing for discrete estimates of the eddy diffusivity of salt KS. Typical plume values of KS are O(10−3 m2 s−1), similar to estimates made in other plume systems and 2 orders of magnitude larger than open ocean eddy diffusivity estimates.