Interdisciplinary Study of Hydrodynamic and Biogeochemical Processes of a Large-Scale River Plume

Abstract

This research has utilised the Massachusetts Institute of Technology gen- eral circulation model (MITgcm) along with observations taken as part of the River In uences on Shelf Ecosystems (RISE) study to investigate the dynamic processes associated with the Columbia River plume at different temporal and spatial scales. Firstly, a high resolution ( x= y=25 m) investigation of the near-field plume was undertaken using the fully non-hydrostatic mode of the MITgcm. This resulted in the reproduction of a detailed inner plume as well as a series of radiated internal waves. In addition to first mode internal waves, second order waves were radiated from the plume boundary when propagation ve- locity becomes sub-critical. Third mode internal waves were also observed, trapped at the plume head. The fine plume structure produced revealed sec- ondary fronts within the plume that also generated internal waves. These features increase the mixing occurring inside the plume, resulting in greater entrainment of underlying waters into the plume. The use of Lagrangian drifters within the model produced detailed results of the recirculation tak- ing place within the emerging plume and how this recirculation changes with depth. This has implications for the near-field recirculation of biologically important solutes present in the plume waters. A second coarser resolution horizontal grid ( x= y=500 m) was imple- mented to investigate the processes of the large-scale plume with the addi- tion of wind forcing. Experiments with both simplified and realistic wind scenarios were carried out and comparisons with in-situ data were made. This revealed the dominance of wind effects on the outer plume and tidal effects on the inner plume. In the simplified wind cases, the classical the- ory of plume propagation under the action of upwelling and downwelling favourable winds was recreated. For the case of realistic winds, there was some success in reproducing a hindcast of the plume location. Tracer fields were used to represent nutrient concentrations based on observed data. Whilst these results showed variations from observations, they did allow a spatially and temporally complete view to be taken of nutrient distribu- tion in the region.