The 'Mad Sea' phenomenon in the Strait of Sicily

Sea level observations at Cape Bon, Tunisia, and Mazara del Vallo, Sicily, show that large, coherent oscillations exist across the Strait of Sicily with significant energy bands centered at periods of 35.3, 41.6, 50.6, 75.8, and 134.5 min, whose possible structure is confirmed by a numerical approximation to the gravitational barotropic normal modes with realistic topography. It is observed that these oscillations are related to the passage of synoptic weather systems over the region. An investigation on the configuration, phase velocity, and direction of approach of atmospheric disturbances over the region suggests that the oscillations in the Strait of Sicily could be forced by instabilities that develop in large-scale, low pressure fronts that propagate as pressure gravity waves with an approximate phase speed between 24 and 30 m s-1.