Observations and generation of internal waves in the Strait of Sicily

Abstract

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 1/48° resolution ENEA model and the 100 m resolution Delft3D model.