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Ocean Science An interactive open-access journal of the European Geosciences Union
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Volume 14, issue 4 | Copyright
Ocean Sci., 14, 801-812, 2018
https://doi.org/10.5194/os-14-801-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 22 Aug 2018

Research article | 22 Aug 2018

Turbulent length scales in a fast-flowing, weakly stratified, strait: Cook Strait, New Zealand

Craig L. Stevens Craig L. Stevens
  • 1National Institute of Water and Atmospheric Research, Greta Point, Wellington, 6021, New Zealand
  • 2Department of Physics, University of Auckland, Auckland, New Zealand

Abstract. There remains much to be learned about the full range of turbulent motions in the ocean. Here we consider turbulence and overturn scales in the relatively shallow, weakly stratified, fast-flowing tidal flows of Cook Strait, New Zealand. With flow speeds reaching 3ms−1 in a water column of  ∼ 300m depth the location is heuristically known to be highly turbulent. Dissipation rates of turbulent kinetic energy ε, along with the Thorpe scale, LT, are described. Thorpe scales, often as much as one-quarter of the water depth, are compared with dissipation rates and background flow speed. Turbulent energy dissipation rates ε are modest but high for oceans, around 5×10−5Wkg−1. Comparison of the buoyancy-limit Ozmidov scale LOz suggest the Cook Strait data lie for the majority of the time in the LOz>LT regime, but not universally. Also, comparison of direct and LT-based estimates of ε exhibit reasonable similarity.

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Mixing in the ocean is highly variable and it is often difficult to measure the more energetic regions. Here we present the first full-depth turbulence profiles from Cook Strait, New Zealand. This 22 km wide channel between the major islands of New Zealand sustains very fast tidally driven flows. The measurements show that large vertical eddies exist, moving water up and down. This will affect stratification, as well as any biology, as it passes through the strait.
Mixing in the ocean is highly variable and it is often difficult to measure the more energetic...
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