Spectra from historic long-term open-ocean moored current meter data between latitudes 0¿ < ∣$varphi$∣ < 45¿ reveal a significant drop in semidiurnal tidal band (D2) energy by ~50% at ∣$varphi$∣ ≈ 25--27¿, whilst the peak near the local inertial frequency f is increased by a factor of ~10 up to the level of D2-energy at ∣$varphi$∣ ≈ 28--30¿, where f coincides with diurnal frequencies. The increase in f-energy is accompanied by a red-shift of the peak frequency to 0.97 ¿ 0.01f, or a poleward spreading of enhanced energy. This contrasts with more common blue-shift. The enhancement may be the result of sub-harmonic instability, as supported by sparse significant bicoherence at half-D2, although i) systematic enhancement of diurnal tidal frequencies, notably M1, was not observed, ii) the latitudes of low D2-energy and high f-energy do not coincide. This may be due to a mix of coupled and independent waves, whilst the poleward trapping of sub-f energy suggests non-traditional effects.