Observed Anisotropy of Spectral Characteristics of Internal Waves Induced by Low-Frequency Currents

Abstract

The isotropy of the internal wave field is investigated from short-time spectra of horizontal current measured in the deep sea. A significant anisotropy is detected at high frequencies during most periods of large mean current. The possible cause of this phenomenon is discussed. A theoretical investigation of Doppler distortion of the measurements shows that the Doppler effect cannot produce the observed anisotropy, which is more likely due to the interaction between internal waves and mean shear currents. Data collected during the MODE-0 experiment suggests this anisotropy over short periods of time is produced by the modulation of the internal wave field by the spatial gradients of low-frequency currents, in good agreement with the theory of Müller. The possible influence of critical layer absorption is briefly discussed.