Observations of shear and vertical stability from a neutrally buoyant float

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 V Z / N > 2 where V z = | V z |) 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 K ρ ∼ 3 × 10 −6 m 2 /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.