TRR 181 Seminar “Structure, dynamics and mixing processes in a buoyant river plume entering a fjord”

The TRR 181 seminar is held by Rebecca McPherson (Universität Hamburg, Postdoc in T3) on November 22, 11 am at Universität Hamburg, Bundesstr. 53, Room 22/23.

Abstract

My PhD thesis seeks to improve the present understanding of the dynamics and mixing processes in the near-field region (first several kilometres) of river plumes discharged into the coastal ocean. To advance this, a controlled buoyant river plume flowing into the head of Doubtful Sound, New Zealand is examined. The inflow is a result of a hydroelectric power station tailrace generating surface speeds of over 2 m/s and maintaining a permanent low-salinity surface layer which results in some of the strongest stratification (N2 = 10-1 s-2) recorded in the coastal ocean. A momentum budget determines the influence of the governing dynamics on near-field plume structure and is quantified using high-resolution observations of the density, velocity and turbulence, and a control volume method. The balance revealed that the deceleration of the plume was controlled by shear-dominated turbulence; however, in the shear-stratified interfacial layer 65% of the total energy was transported out beyond the plume's boundaries by internal waves. Observations of turbulent mixing were quantified using direct measurements of turbulence dissipation rates (ε) and indirect estimates of ε from turbulent overturn analysis (Thorpe scales). The dissipation rates in the near-field plume region (ε > 10-2 W/kg) are amongst the highest recorded in oceanic shear flows.