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Discussion papers
https://doi.org/10.5194/os-2019-120
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/os-2019-120
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 27 Nov 2019

Submitted as: research article | 27 Nov 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Ocean Science (OS).

The role of turbulence and internal waves in the structure and evolution of a near-field river plume

Rebecca A. McPherson1, Craig L. Stevens1,2, Joanne M. O'Callaghan2, Andrew J. Lucas3, and Jonathan D. Nash4 Rebecca A. McPherson et al.
  • 1Department of Physics, University of Auckland, Auckland, New Zealand
  • 2National Institute of Water and Atmospheric Research, Wellington, New Zealand
  • 3Scripps Institution of Oceanography, University of California San Diego, USA
  • 4College of Earth, Oceans and Atmospheric Sciences, Oregon State University, USA

Abstract. An along-channel momentum budget is quantified in the near-field plume region of a controlled river flow entering Doubtful Sound, New Zealand. Observations include highly resolved density, velocity and turbulence, enabling a momentum budget to be constructed over a control volume. Estimates of internal stress (τ) were made from direct measurements of turbulence dissipation rates (ε) using vertical microstructure profiles. High flow speeds of the surface plume over 2 m s−1 and strong stratification (N2 ~ 10−1 s−2) resulted in enhanced turbulence dissipation rates (ε > 10−3 W kg−1) and internal stress (τ > 10−2 m2 s−2) at the base of surface layer. An observed transition from a supercritical to sub-critical flow regime in the initial 1 km indicates the presence of an internal hydraulic jump and the subsequent release of internal gravity waves. The momentum flux divergence of these internal waves suggests that almost 15 % of the total plume momentum can be transported out of the system by wave radiation, therefore playing a crucial role in the redistribution of momentum within the near-field plume. Observations illustrate that the evolution of the momentum budget components vary between the distinct surface plume layer and the turbulent, shear-stratified interfacial layer. Within the surface plume, a momentum balance was achieved. The dynamical balance demonstrates that the deceleration of the plume, driven by along-channel advection, is controlled by turbulence stress from the plume discharge point to as far as 3 km downstream. In the interfacial layer however, the momentum equation was dominated by the turbulence stress term and the balance was not closed. The redistribution of momentum within the shear-stratified layer by the observed hydraulic jump and internal wave radiation could account for the discrepancy in the budget.

Rebecca A. McPherson et al.
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Rebecca A. McPherson et al.
Rebecca A. McPherson et al.
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Short summary
River plume characteristics (density, velocity, turbulence) are measured in the first several kilometers of a river flow entering Doubtful Sound, New Zealand. These observations are used to examine the momentum of the plume and determine the influence of the main dynamics on controlling the behavior and structure of the plume. The consideration of internal hydraulics and wave radiation is necessary to understand fully the impact of governing dynamics on plume behavior and evolution.
River plume characteristics (density, velocity, turbulence) are measured in the first several...
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