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L. Gostiaux and H. van Haren

Fine-structure contamination by internal waves in the Canary Basin

J. Geophys. Res., 117, C11003 (2012)

Over a range of 132.5m, 54 temperature sensors (1mK relative accuracy) were yearlong moored while sampling at 1Hz around 1455m 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<100m, including at sensor interval \Delta z=2.5m. Instead of a dominant 0-phase difference over the range of sensors, as observed for internal waves at frequencies f<\sigma<N, f denoting the inertial frequency, this super-buoyancy coherence shows \pi-phase difference. The transition from 0-phase difference, for internal waves, to \pi-phase difference is abrupt and increases in frequency for decreasing \Delta z<10m. For \Delta z>10m, the transition is fixed at N_t\simeq 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 non-linearities in the temperature gradient, such as steps in the temperature profile, are advected passed 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 like -8/3. The -8/3 slope can however be reproduced in our model, provided the jumps in the temperature profile are not infinitely thin.

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