Vertical Coherence of the Internal Wave Field from Towed Sensors

Constant depth and isopycnal-following tows are used to estimate the towed vertical coherence of the internal wave field, at vertical separations of 8.5, 18, 28, and 70 m. The depths of the tows are similar to 750 m at the maximum of the buoyancy frequency in the main thermocline of the Sargasso Sea, and near 350 m in the buoyancy frequency minimum between the main and seasonal thermoclines. The towed spectra and towed vertical coherence are compared with three model spectra (GM75, GM76, and IWEX); at 750 m, the agreement between data and models is good, with IWEX being slightly better. At 350 m, several of the measured, towed, vertical coherence spectra are more complex than the spectra from the deeper tows; there are anomalously high coherences, in a band from 0.7 to 2 cycles/km, which are not predictable by the models. This coherence bump may be evidence of Eckart resonance, i.e., modes tunneling between the two thermoclines into the region of low buoyancy frequency.