Shear at the critical diurnal latitude

Around latitudes ∣ϕ∣ ≈ 30° where diurnal D 1 equals the local inertial frequency f = 2Ωsinϕ, Ω denoting the Earth's rotational vector, several mechanisms can enhance shear at f due to a reduction in vertical scales. This would imply locally enhanced deep‐ocean mixing. Here, recent 1.5 years of acoustic Doppler current profiler (ADCP) observations from the Canary Basin demonstrate largest kinetic energy at semidiurnal tides (D 2 ), but a complete absence of D 2 ‐shear. Instead, shear is peaking at subinertial 0.97 ± 0.01f and terdiurnal 3f(≈D 2 + f ≈ D 3 here), and vertical scales Δz(f) < 0.1Δz(D 2 ). However, the f‐band is broader than deterministic tidal frequencies and the smallest vertical scales, organizing shear in thin layers, are found at the lower inertio‐gravity wave limit, which equals 0.97f for the weakest stratification observed (N = 6f, using Δz = 10 m). Hence, besides possibly subharmonic resonance, other mechanisms must be involved in enhancing f‐shear, including non‐linear harmonic interactions and wave trapping at the critical latitude's poleward shift.