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Observations of shear and vertical stability from a neutrally buoyant float

Authors

  • Eric Kunze,

  • A. J. Williams III,

  • Melbourne G. Briscoe


Abstract

Measurements of 1- to 5-m shear and strain from a neutrally buoyant float are used to examine the statistics and causes of shear instability in a 9-day record. “Unstable” conditions (defined as VZ/N > 2 where Vz = |Vz|) are due to higher-than-average shear rather than lower-than-average buoyancy frequency and there are no instances of unstable buoyancy frequency, or overturns, on the ≥0.5-m spacing of the thermistors. Shear is dominated by upward propagating near-inertial motions. Unstable events were more frequent when a near-inertial wave packet occupied the water parcel tracked by the float. Groups of unstable events occur roughly every 5–7 hours at scales <2.5 m and much less often at larger scales. Events typically last 10 min (one buoyancy period) or longer. These time scales appear to be controlled by vertical advection of ∼1-m fine structure past the float sensors so do not represent the intrinsic time scales of instability. Turbulent and mixing quantities are estimated from these fine-scale measurements, giving a dissipation rate of ε ∼ (2.3–16) × 10−10 W/kg and an eddy diffusivity of Kρ ∼ 3 × 10−6 m2/s, in agreement with direct microstructure estimates. Therefore the float adequately resolves those quantities needed to estimate turbulent dissipation and could be used to monitor turbulent mixing in the ocean.

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