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 Delta z < 100 m, including at sensor interval Delta z = 2.5 m. Instead of a dominant zero-phase difference over the range of sensors, as observed for internal waves at frequencies f < sigma < N, with f denoting the inertial frequency, this superbuoyant coherence shows pi-phase difference. The transition from zero-phase difference, for internal waves, to pi-phase difference is abrupt and increases in frequency for decreasing Delta z < 10 m. For Delta z > 10 m, the transition is fixed at N-t similar or equal to 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 above N 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