High-frequency internal wave motions at the ANTARES site in the deep western mediterranean

Abstract

[EN] High-frequency internal wave motions of periods down to 20 min have been observed in a yearlong record from the deep Western Mediterranean, mainly in vertical currents. The observations were made using the ANTARES neutrino telescope infrastructure. One line of the telescope is instrumented with environmental monitoring devices, and in particular with an Acoustic Doppler Current Profiler that was used to measure currents around 2,200 m. Such high-frequency internal waves are commonly observed much closer to the sea surface where the vertical density stratification is more stable than in the deep sea. In this paper, they are supported by the relatively large stratification following newly formed dense water. During the severe winters of 2005 and 2006, deep dense-water formation occurred in the Ligurian subbasin. Its collapse and spread over the sea floor across the basin remained detectable for at least 3 years as deduced from the present yearlong current record, which is from 2008. The observed high-frequency internal waves match the occasional density stratification observed in ¡«1-m-thin layers using previous shipborne conductivity¿C temperature¿C depth measurements. Such layers and waves are relatively unusual in the deep Mediterranean, where commonly several hundreds-ofmeters- thick near-homogeneous layers dominate. Such thick near-homogeneous layers provide about a half-decade narrow internal wave band around the inertial frequency (f). In contrast, the presently observed vertical currents occasionally show a ¡°small-scale¡± internal wave band that is on average 1.5 decades wide, associated with thin-layer stratification. In spite of its relatively largewidth, this band still shows variance peaking near f rather than near the large-scale buoyancy frequency N (= 2.3 4.5f) and this variance is found to increase with increasing N.