Bandwidth similarity at inertial and tidal frequencies in kinetic energy spectra from the Bay of Biscay

Abstract

Two contrasting mechanical sources may generate internal gravity waves in the ocean: spectrally sharp and spatially localized tide-topography forcing, and widespread and spectrally broad atmospheric forcing. However, their responses were observed very similar in yearlong current observations down the continental slope into the abyssal plain of the Bay of Biscay. This similar response (around semidiurnal tidal and local inertial frequency, respectively) was neither spectrally sharp-like purely deterministic barotropic tidal forcing nor broad-like atmospheric forcing. Instead, it had a limited spectral bandwidth Δ σ=(0.09±0.02) σ, σ denoting frequency, which determined ‘intermittency’. This similar relative bandwidth in frequency was attributed to ocean response being less dependent on external forcing and primarily dependent on variations in internal wave background conditions, such as stratification, shear, effective inertial frequency, and non-linear interaction. The same relative bandwidth was also found around super-tidal frequencies within the internal wave band associated with inertial–tidal and higher tidal harmonics, because of (i) the above narrow-band character of near-inertial motions in which the initial response to atmospheric disturbances was ‘focused’, and (ii) the redistribution of inertial and tidal energy to other frequencies through non-linear interactions.