Fine-structure contamination by internal waves in the Canary Basin
Résumé
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$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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