Spatial and Temporal Variability in Vertical Mixing in the Core of a Tidally Energetic River Plume

Field observations were collected in the Connecticut River plume to investigate the temporal evolution of vertical mixing and transport between the plume core and the plume front. The goal of this work is to improve the understanding of the length scales associated with frontal mixing and transport, and their connection to the outflow, from field data. The Connecticut River plume is tidally pulsed and rotates from east to west as it exits the mouth with a complex outflow geometry. Sampling began when the buoyant outflow turned to the west during plume formation (i.e. ebb to flood transition). Three streamwise transects (starting 5, 6.75, and 9.75 hrs. after high water) were collected following the westward plume and spanned from the river mouth to the plume front during relatively low (~140 m³/s) river discharge. These are the first observations to track the evolution of turbulence in the near and mid-field plume regions spanning from the source to the bounding front. Measurements were made with a surface mounted 1200 kHz Acoustic Doppler Current Profiler (ADCP) and Acoustic Doppler Velocimeter (ADV) measuring current velocities continuously. Velocity measurements were complemented with data from a MicroCTD in upriser mode allowing for microstructure estimates of stratification and turbulence collected at the river mouth and moving stations at 50%, 75%, 90%, and 110% of the plume length (L). These observations were accompanied by drifter data identifying the plume front. Through the sampling period, turbulent kinetic energy dissipation rates, ε, at the plume base (~2 m depth) increased at the source region (from 10^-6 to 10^-5 W kg^-1) but remained constant in the plume interior (10^-6 W kg^-1). Enhanced subsurface ε (10^-5 W kg^-1 at 6 - 9 m) evolved from the outer plume region (90%L) to encompass the entire plume area as the ambient currents increased with flood tide.