Observations and generation of internal waves in the Strait of Sicily

Résumé fourni par la source

Oceanic internal solitary waves (ISWs) can be generated by several mechanisms, among which by tidal flow over shallow bathymetry, atmospheric forcing, unbalanced sub-mesoscale flow without external forcing and rapid displacements of the sharp fronts separating e.g. cold ocean water masses or river plumes from the ambient seawater. In this paper, we analyze synthetic aperture radar (SAR) images acquired by the Sentinel-1 satellites over the Adventure Bank area, located on the western side of the Strait of Sicily in the years 2018-2021 showing numerous radar signatures of ISWs. This is the first time that massive presence of ISWs is reported in this area of the Mediterranean Sea. The internal wave field, observed primarily in the months from May to September, is very heterogeneous, with wavelengths ranging from a few hundred meters to a few kilometers, wave front lengths from a few km to about 40 km, and propagating in almost any direction. This suggests that the internal waves detected in the SAR images are generated by a variety of mechanisms and not by a specific one. Although in general tidal currents are weak in the Mediterranean Sea, the sum of the tidal flow and the Atlantic-Ionian Stream current can attain values up to 0.3 m s -1 such that ISW generation by interaction with shallow underwater bottom topography becomes possible. Furthermore, cold filaments and sub-mesoscale cyclonic eddies generated by upwelling at the south coast of Sicily, feature sharp and rapidly moving fronts, from which ISWs may originate. Theoretical evidence for these two types of ISW generation mechanisms is provided by model results obtained from the low-resolution ENEA model and the high-resolution Delft3D model. • Internal Solitary Wave (ISW) signatures have been observed in SAR images of the Strait of Sicily, over the Adventure Bank (AB), at the Strait's western end • the AB area is characterized by rugged bathymetry, and strong currents (Atlantic Ionian Stream) and frequent coastal upwelling generating offshore filaments and sub-mesoscale eddies in the warm season. • These features cause ISW generation by both flow-bathymetry interaction and frontal dynamics at the filament and eddy margins, as results for the first time, from investigation with SAR and Sea Surface Temperature satellite imagery and numerical models.

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