Fine‐structure contamination by internal waves in the Canary Basin

Over a range of 132.5 m, 54 temperature sensors (1 mK relative accuracy) were moored yearlong while sampling at 1 Hz around 1455 m in the open Canary Basin. Coherence between individual records shows a weak but significant peak above the local buoyancy frequency N for all vertical separations Δz < 100 m, including at sensor interval Δz= 2.5 m. Instead of a dominant zero‐phase difference over the range of sensors, as observed for internal waves at frequenciesf < σ < N, with f denoting the inertial frequency, this superbuoyant coherence shows π‐phase difference. The transition from zero‐phase difference, for internal waves, toπ‐phase difference is abrupt and increases in frequency for decreasing Δz < 10 m. For Δz > 10 m, the transition is fixed at Nt ≃ 1.6N, which is also the maximum value of the small‐scale buoyancy frequency, and limits the internal wave band on its high‐frequency side. In the time domain it is observed that this high‐frequency coherence mainly occurs when nonlinearities in the temperature gradient, such as steps in the temperature profile, are advected past the sensors. A simple kinematic model of fine‐structure contamination is proposed to reproduce this observation. The canonical −2 slope of the temperature spectrum aboveN is not observed in the in situ data, which rather slope as −8/3. The −8/3 slope can, however, be reproduced in our model, provided the jumps in the temperature profile are not infinitely thin.